VCE Chemistry Question Thread

[RazzMeTazz]

For this question from the heinemann textbook:
During the preparation of the standard solution in volumetric analysis, why is water added to the level of the calibration mark on the flask after the solid has dissolved, rather than before?

The worked solutions says:
When a substance is dissolved, there is often a slight change in volume due to the attractions between the solute and solvent particles. Since molar concentration is measured in mole of solute per litre of solution, it is necessary to accurately measure the volume of solution rather than the volume of water used.

Could you also say: When trying to make a specific volume of a standard solution, you add the solid first and then water to the level of the calibration mark on the flask. This is because you do not know the density of the solid - so if you added the solid after you added the water, your volume may be altered and you will not achieve your desired volume of standard solution.

  Thanks

[Delirium]

How does the equation ammonium ion (NH4+) and sodium hydroxide (NaOH+) look like?
Thanks

[Chang Feng]

How would you approach finding all the isomers of a specific molecular formula?? Is there any fast way of doing it or can you only just draw out all possibilities to find the answer.

[paper-back]

How is the reaction; NaH+H2O->Na+ + OH- + H2 an acid-base reaction?

[keltingmeith]

How is the reaction; NaH+H2O->Na+ + OH- + H2 an acid-base reaction?

Sodium Hydride lost a proton, hence it is an acid by the Lowry-Bronsted definition, and this is an acid-base reaction.

This might not seem like an acid-base reaction (because it's not acid + base = salt + water), but the proton exchange means it's a type of acid-base reaction. (inb4 someone says "Lewis acid")

How would you approach finding all the isomers of a specific molecular formula?? Is there any fast way of doing it or can you only just draw out all possibilities to find the answer.

Draw them all. I think there's a formula for alkanes that gives you the number of all the structural isomers, but I personally think learning it is a waste of time.

How does the equation ammonium ion (NH4+) and sodium hydroxide (NaOH+) look like?
Thanks

Assuming you mean for when you react them together:

For this question from the heinemann textbook:
During the preparation of the standard solution in volumetric analysis, why is water added to the level of the calibration mark on the flask after the solid has dissolved, rather than before?

The worked solutions says:
When a substance is dissolved, there is often a slight change in volume due to the attractions between the solute and solvent particles. Since molar concentration is measured in mole of solute per litre of solution, it is necessary to accurately measure the volume of solution rather than the volume of water used.

Could you also say: When trying to make a specific volume of a standard solution, you add the solid first and then water to the level of the calibration mark on the flask. This is because you do not know the density of the solid - so if you added the solid after you added the water, your volume may be altered and you will not achieve your desired volume of standard solution.

  Thanks

I think that what you're saying is right, but it's making me scratch my head. You don't want to do that to someone assessing you - if your goal is to try super smart and clever, don't. A simple answer is just as good as a complex answer (in fact, better) if it's right.

[nick_9_8]

Question regarding significant figures:


If the molar mass of a substance I get off the periodic table has 3 significant figures, and the rest of my data used in my calculations has 4 significant figures, what do I round the final answer to? 3sf or 4sf?

Additionally, could someone provide some VCAA documentation on the correct answer, as I've heard multiple opinions but am unsure how vcaa marks it.

Cheers

[lzxnl]

Sodium Hydride lost a proton, hence it is an acid by the Lowry-Bronsted definition, and this is an acid-base reaction.

This might not seem like an acid-base reaction (because it's not acid + base = salt + water), but the proton exchange means it's a type of acid-base reaction. (inb4 someone says "Lewis acid")

I'm not going to say 'Lewis acid'. I'm going to say...sodium hydride lost a proton? What proton?
The hydride ion, H-, is the smallest anion possible (aside from the electron itself). Such a large charge density is terrific at grabbing protons; it's an extraordinarily powerful base. Its conjugate 'acid' is H₂, which isn't acidic at all.
You can consider the hydride ion as a Bronsted base as it does attract H+ to form molecular hydrogen; I think of it this way at least.

Question regarding significant figures:


If the molar mass of a substance I get off the periodic table has 3 significant figures, and the rest of my data used in my calculations has 4 significant figures, what do I round the final answer to? 3sf or 4sf?

Additionally, could someone provide some VCAA documentation on the correct answer, as I've heard multiple opinions but am unsure how vcaa marks it.

Cheers

3 sig figs is what I'd go with. You're using data that is only correct to 3 sig figs in your working.

[Kalopsic]

Question regarding significant figures:


If the molar mass of a substance I get off the periodic table has 3 significant figures, and the rest of my data used in my calculations has 4 significant figures, what do I round the final answer to? 3sf or 4sf?

Additionally, could someone provide some VCAA documentation on the correct answer, as I've heard multiple opinions but am unsure how vcaa marks it.

Cheers

I've asked this same question to many tutors (from TSFX) and teachers. Everyone except my teacher, funnily enough, said your answer needs to have the same amount of sig figs as the number with the lowest sig figs used in your calculations. However, the rule is different for pH's.

[keltingmeith]

I'm not going to say 'Lewis acid'. I'm going to say...sodium hydride lost a proton? What proton?

Urp, my bad, that hydroxide on the RHS should've slapped me in the face more. >.>;

[That Other Guy]

I just did my first SAC: the generic back titration with lawn fertilizer.
Just a quick question regarding the use of indicator. We used methyl red. Why did we not use phenolphthalein? I have a feeling that it's to do with the fact that the ammonium sulphate in the fertilizer acts as a weak acid and therefore does not elicit a sharp enough endpoint... but I can't think of a sound answer. Any ideas?

[thushan]

If anything, I think it's because there is ammonium lurking around in the reaction mixture after you dump NaOH and back titrate against HCl. Therefore, at equivalence point, there are some ammonium ions running about, meaning the pH at equivalence point would be less than 7, making phenolphthalein an undesirable indicator to use.

[Kel9901]

If anything, I think it's because there is ammonium lurking around in the reaction mixture after you dump NaOH and back titrate against HCl. Therefore, at equivalence point, there are some ammonium ions running about, meaning the pH at equivalence point would be less than 7, making phenolphthalein an undesirable indicator to use.

I think you boil off the ammonia produced to make the pH of the equivalence point exactly 7 (as eq. point of NaOH and HCl is pH=7). My school used phenolphthalein when I did it last year

[lzxnl]

I think you boil off the ammonia produced to make the pH of the equivalence point exactly 7 (as eq. point of NaOH and HCl is pH=7). My school used phenolphthalein when I did it last year

You do need to be careful with your boiling to ensure ALL the ammonia is boiled off.

[RazzMeTazz]

The answer to this question is A, however I don't really understand why since I thought metal oxide + acid ---> salt + water

Thanks in advance

[RazzMeTazz]

I didn't really understand how to go about question 95.) and 83.) ( both attached below)

Any insight/explanations would be great!