Chemistry HL/SL help

[Morgan1105]

Hi guys! Thanks for being so kind and helping us. I am only in Chemistry Pre- IB but i do have a question:

So we are being asked to calculate [H3O+] when given the amount of moles. However, my teacher only tought us how to find [OH-] and [H+]. How would i go about finding the [H30+] ? Are [H3O+] and [H+] the same then?

Edited March 31, 2013 by Morgan1105

[Emmi]

Hi guys! Thanks for being so kind and helping us. I am only in Chemistry Pre- IB but i do have a question:

So we are being asked to calculate [H3O+] when given the amount of moles. However, my teacher only tought us how to find [OH-] and [H+]. How would i go about finding the [H30+] ? Are [H3O+] and [H+] the same then?

Yes, H₃O⁺ and H⁺ are the same thing. H₃O⁺ (the hydronium ion) technically is the correct form, because when water is protonated, we get the reaction 2H₂O <---> OH^- + H₃O⁺. H⁺ is just the shorthand version of writing the hydronium ion because chemists are lazy

[Morgan1105]

Awesome!!!! I totally understand! Thank you

[manav]

Can anyone please suggest me some good study material (other than the textbook) for the options in chemistry

-Modern analytical.

-Biochemistry.

[bluedino]

Well I don't know what textbook you have, but I'm going to suggest the one I use? I use the Pearson Baccalaureate Chemistry Options Textbook (Derry, et al), which I find really simple, clear and useful. Obviously I don't know if you learn the same way as I do, but I can basically just learn everything from my textbook and be assured that it covers all the syllabus points.

Again, depending on how you learn, the Chemistry IB Study Guide by Geoffrey Neuss might be helpful for you - it summarises topics (including the options) and there are a few exam questions. There are a few other study guides (if you just Google you'll find them), but I've never used them so I can't say how useful they might be. There's also a lot of notes and resources that you can find online, one example being +Wikibooks - follow the links on that page, and then there are more links within the options themselves. Not sure how reliable and accurate those notes might be though. Maybe some of the links on +this page might also be helpful.

The most important thing for me is doing past exam papers or just some past questions - such as from Questionbank.

Another crucial study material for me is the Syllabus. It is seriously imperative to go through the syllabus points and make sure you know each of them as that is what the IBO will assess you on.

Basically, I don't know how you like to study, but personally, I like to have the syllabus next to me while I read through the textbook learning/revising it all, then I do past papers and questions, and that's it. I know one or two people who really like to use Quizlet to study - for example, making flashcards of all the definitions you need to know and all the things that you need to describe, etc., or hand making flashcards with diagrams of things? It might be useful for Biochemistry. Hope I helped

Edited May 18, 2013 by bluedino

[procrasti(nation)^2]

I need some help with this one question -

What is the pH of the resulting solution when 20.00 cm³ of 0.20 M HCl is added to 100.00 cm³ of 0.10 M NH₃(aq)? The pK_b of NH₃ is 4.75.

Please and Thank You!!

[AlexW]

Hey guys

I have an assigment from stichiometry and i can't do one problem.

Anhydrous copper sulfate is a white powder with the formula CuSO4. A group of students placed a weighed sample of blue copper sulfate into a beaker and heated slowly until it became white. When cool they weighed the chemical again and found that it had lost 36.0% of it s mass. They assumed that the drop in mass was all due to the evaporation of water of cristallisation from the copper sulfate crystals. Using this information, determine the molecular formula of the original hydrated copper sulfate crystals.

Thanks in advance.

[Emmi]

Hey guys

I have an assigment from stichiometry and i can't do one problem.

Anhydrous copper sulfate is a white powder with the formula CuSO4. A group of students placed a weighed sample of blue copper sulfate into a beaker and heated slowly until it became white. When cool they weighed the chemical again and found that it had lost 36.0% of it s mass. They assumed that the drop in mass was all due to the evaporation of water of cristallisation from the copper sulfate crystals. Using this information, determine the molecular formula of the original hydrated copper sulfate crystals.

Thanks in advance.

The anhydride (the sulfate without water attached to it) has a formula of CuSO₄. The formula of the hydrate (the blue copper (II) sulfate) is CuSO₄ ∙ xH₂O. Your job is to find x, which will give you the molecular formula of the original hydrated compound.

Since they don't give you a specific mass, you are free to assume your own mass. In this case, it's easiest to assume that you start with a 100.0 gram sample of the hydrate, because then it makes doing stoichiometry calculations very easy.

It says the compound lost 36% of its mass after heating. This means that in the original 100.0 gram sample, there were 36.0 grams of water. The remaining 64.0 grams must have been the copper (II) sulfate. To find how many moles of each you had:

36.0 grams H₂O * (1 mole H₂O / 18.106 grams H₂O) = 1.998 moles H₂O

64.0 grams CuSO₄ * (1 mole CuSO₄ / 159.62 grams CuSO₄) = 0.4009 moles CuSO₄

You now divide by the smallest number you received above; in this case it would be the moles of copper (II) sulfate. Dividing each number of moles by the smallest number will give you how many moles were in the original compound:

0.4009 moles CuSO₄ / 0.4009 moles = 1 mole CuSO₄

1.998 moles H₂O / 0.4009 moles = 4.98 moles = 5 moles H₂O

Thus the molecular formula of the original compound was CuSO₄ ∙ 5H₂O.

Edited September 22, 2013 by Emmi

[EmmmaD]

Hi!

I am doing chem SL and having trouble with redox equations...

Could someone explain them to me?!

Thanks

[Emmi]

Hi!

I am doing chem SL and having trouble with redox equations...

Could someone explain them to me?!

Thanks

A "redox" reaction is short for an oxidation-reduction reaction. When one of these happens, you'll see an change in the oxidation numbers of different elements. An oxidation number tells you about the electronic character of that element. An atom in its neutral state has an oxidation state of 0. For ions and compounds, you look at its charge either by itself or within the compound. For example, the magnesium in Mg²⁺ has an oxidation number of +2. Certain elements tend to always have the same oxidation number, like the Group 1 and 2 elements, the halogens, and things like aluminum and hydrogen, but others change based on what compound they are in. For example, copper's oxidation state in CuSO₄ is +2, but in CuI it's +1. This happens a lot with transition metals and is why you see a lot of different colors for solutions of them and why they're good catalysts (don't know if you need this for SL, but it's still interesting).

Lots of reactions are actually redox reactions, it's just sometimes not that obvious. For example, the reaction 2HCl + Li --> 2LiCl + H₂ is actually a redox reaction although most would simply classify it as a single replacement or displacement reaction because the oxidation states change. Hydrogen goes from a +1 to a 0 oxidation state, so it is reduced. A reduction occurs when an element gains elections, which means its oxidation state decreases. This kind of doesn't make sense, because why would something that gains electrons be reduced, but it makes perfect sense when you think about it, because when something gains electrons, it becomes less positive. On the other hand, lithium goes from a 0 to a +1 oxidation state, so it is oxidized. An oxidation occurs when an element loses electrons, which gives it an increase in oxidation state (it's becoming more positive). An element can get oxidized in either direction, i.e. O₂ can get oxidized to form O^2-, it doesn't always form a positive ion.

When you write and balance redox equations, the charges on each side of the equation have to match up (i.e. if you started with an overall net charge of 0 on one side, the other side must be 0 as well). It's best to do this in half-reactions where you look at each element individually, and them sum them together.

Hope this clears things up a bit

[EmmmaD]

Hi!

I am doing chem SL and having trouble with redox equations...

Could someone explain them to me?!

Thanks

A "redox" reaction is short for an oxidation-reduction reaction. When one of these happens, you'll see an change in the oxidation numbers of different elements. An oxidation number tells you about the electronic character of that element. An atom in its neutral state has an oxidation state of 0. For ions and compounds, you look at its charge either by itself or within the compound. For example, the magnesium in Mg²⁺ has an oxidation number of +2. Certain elements tend to always have the same oxidation number, like the Group 1 and 2 elements, the halogens, and things like aluminum and hydrogen, but others change based on what compound they are in. For example, copper's oxidation state in CuSO₄ is +2, but in CuI it's +1. This happens a lot with transition metals and is why you see a lot of different colors for solutions of them and why they're good catalysts (don't know if you need this for SL, but it's still interesting).

Lots of reactions are actually redox reactions, it's just sometimes not that obvious. For example, the reaction 2HCl + Li --> 2LiCl + H₂ is actually a redox reaction although most would simply classify it as a single replacement or displacement reaction because the oxidation states change. Hydrogen goes from a +1 to a 0 oxidation state, so it is reduced. A reduction occurs when an element gains elections, which means its oxidation state decreases. This kind of doesn't make sense, because why would something that gains electrons be reduced, but it makes perfect sense when you think about it, because when something gains electrons, it becomes less positive. On the other hand, lithium goes from a 0 to a +1 oxidation state, so it is oxidized. An oxidation occurs when an element loses electrons, which gives it an increase in oxidation state (it's becoming more positive). An element can get oxidized in either direction, i.e. O₂ can get oxidized to form O^2-, it doesn't always form a positive ion.

When you write and balance redox equations, the charges on each side of the equation have to match up (i.e. if you started with an overall net charge of 0 on one side, the other side must be 0 as well). It's best to do this in half-reactions where you look at each element individually, and them sum them together.

Hope this clears things up a bit

Oh my gosh, thank you so much!

This really helped

[ElvenRanger]

Hi everyone, my chem teacher just tried to teach us titrations in one class and we're all confused now. What are your suggestions for understanding how these work?

[Emmi]

Hi everyone, my chem teacher just tried to teach us titrations in one class and we're all confused now. What are your suggestions for understanding how these work?

Understand what's going on based on what type of reactants you use. There's four cases: strong acid/strong base, strong acid/weak base, weak acid/strong base, and weak acid/weak base. Each of these has a different titration curve that will tell you about their starting pHs, pKas, equivalence points, and ending pHs.

An indicator exists in equilibrium and works by shifting to one side or the other of its equilibrium depending on the concentration of your reactants. For example phenolphthalein exists as a clear solution when the pH is less than the equivalence point, a light pink solution at the equivalence point, and a dark fuchsia color when the pH is higher than at the equivalence point. Different indicators work better for different scenarios, you should look at where the expected equivalence point will occur and pick an indicator that shifts equilibrium in this range.

To solve titration type problems, use the Henderson-Hasselbalch equation (pH = pKa + log([A^-]/[HA]) where A- is the concentration of the anion that exists in equilibrium, and HA is the concentration of the acid at equilibrium. You can also solve these through stoichiometry. The trick to being good at these is to do lots of examples.

[Eman Salem_121865]

Hello!

I need some help because I have to pick 6 labs to do for my final IAs being submitted and I don't know which ones are the best for IB.

I am wondering if you have any suggestions.

Please and thank you!

[Emmi]

Hello!

I need some help because I have to pick 6 labs to do for my final IAs being submitted and I don't know which ones are the best for IB.

I am wondering if you have any suggestions.

Please and thank you!

Any lab you can do on a topic in the syllabus is fine. It doesn't matter what you choose, as long as you represent every marking criteria twice. This means you can submit as few as two labs (each lab was marked on design, data collection, and conclusion/evaluation) or as many as six (each lab was marked only in one category). Ideally they should be the labs you scored highest on.

[caliefleurette]

Hello! just a little question...

So I am given the exponential curve of a graph that has its maximum close to the y-axis and the minimum close to the x-axis of a fixed amount of gas.

I have to choose what variables the graph is representing

a) x-axis pressure y-axis temperature

b) x-axis volume y-axis temperature

c) x-axis pressure y-axis volume

d) x-axis temperature y-axis volume

... i'm kind of lost because all the diagrams i ever saw had pressure as y-axis and volume as x-axis...

Could anyone help please?

Thanks a lot!

[ib1123]

@caliefleurette

Assuming you're referring to an exponential decay curve shape, the answer should be C. I recommend you draw each scenario out and evaluate the plausibility of each pair of axes.

For option A, if you label the axes as given (i.e. pressure on x-axis and temperature on y-axis), then the implication is that pressure increases as temperature drops. However, if you have a container full of gas and increase the temperature, you're simultaneously increasing the energy of the constituent particles. Thus these particles want to move around more, and greater pressure would consequently be exerted on any container walls. The same principle applies for an open container situation, but it's just easier to conceptualise with a closed container.

Option B would suggest that as you increase temperature, the volume would decrease. The exact opposite is true. By adding more energy (via increased temperature) the constituent gas particles will have more energy to more around and thus will expand, leading to increased volume.

Regarding option C, the implication is that as you increase the pressure, the volume will decrease. This is true, since increasing pressure will compress the gas and thus decrease volume. Hence, this option is correct.

As for option D, this axes labelling suggests that increasing temperature will decrease the volume. However, for the same reason that option B is incorrect, so too is option D incorrect. Increasing temperature will expand the gas since it provides the gaseous particles with more energy to move around.

In terms of a more general approach to studying for these types of Chemistry questions - I don't recommend learning specific graphs. Rather, it will be far more beneficial to learn the principles underlying the drawing of said graphs and labelling of their axes.

[caliefleurette]

Thanks so much! Also i'm not sure about this question:

How many protons contained in 1.00 g of chlorine gas?

so i did

RAM=35

1.00g

1/35=0.03

6.023x10^23x0.03= 8.42x10^18

but is it Cl₂ so therefore

1/70=0.014

6.023x10^23x0.014= 8.4322x10^21

Which one is the correct answer?

Thanks to all!

[caliefleurette]

Another question...

How can you simply make elemental bromine, Br2, from the compound NaBr?

I don't understand how to just "know" the answer to these questions..

I thought about putting some chlorine in there to make NaCl but that would make a huge reaction right?

Thanks!

[ib1123]

Thanks so much! Also i'm not sure about this question:

How many protons contained in 1.00 g of chlorine gas?

so i did

RAM=35

1.00g

1/35=0.03

6.023x10^23x0.03= 8.42x10^18

but is it Cl₂ so therefore

1/70=0.014

6.023x10^23x0.014= 8.4322x10^21

Which one is the correct answer?

Thanks to all!

The way you've written your calculations, for both scenarios, is correct. However, you've made a computational error in both cases. The numbers you should get instead of 8.42x10^18 and 8.4322x10^21 are 1.7209x10^22 and 8.6043x10^21 respectively.

Now, whether you assumed the chlorine gas was monoatomic or diatomic, you should end up with the same answer overall. The issue you have is that neither 1.7209x10^22 or 8.6043x10^21 represent the number of protons; they represent the number of molecules in that 1g.

Thus, for the first set of calculations:

Multiply the final answer you get using your method so far (this number should be 1.7209x10^22) by 17. This is because each monoatomic chlorine molecule will contain 17 protons. I got this number from the periodic table, since the atomic number of chlorine is 17. Your resultant answer should be approximately 2.9x10^23.

For the second set of calculations:

Multiply your final number using your method (this number should be 8.6043x10^21) by 34. This is because each diatomic chlorine molecule contains 34 protons (since there's two chlorine atoms, each with 17 protons). The resultant answer should again be 2.9x10^23.

If you're repeating your calculations and are getting answers which are slightly off, then this is probably due to a rounding error. So, instead of doing the calculations in multiple steps, do it all in one. e.g. for your first set of calculations, type into your calculator:

(1/35) x (6.023) x (10^23) x (17)