HSC Chemistry Question Thread

[jakesilove]

Hey guys!
Two beakers X and Y; either contains 0.1mol/L of HCL or 0.1mol/ acetic acid. A student performs two tests:
test 1: stuck a probe metre in to determine the pH
test2: add a volume of o.1mol/L of NaOH needed to neutralise each acid and using phenolpthalein indicator.
Assess the effectiveness of determining which acid is in which beaker when performing the two tests.
I was able to do test 1 but I need help for test 2. If HCL + NaOH--> NaCl + H2O then wouldn't the solution turn colourless and if CH3COOH + NaOH --> CH3COONa + H2O be purple (slightly basic salt?)
Thanks

Hey! Definitely a seriously weird question, with loads of ways you could interpret it. In one, we're titrating a strong acid with a weak base. In the other, we're titrating a weak acid with a strong base. Phenophthalein will only be appropriate for the first titration, so maybe the question is getting you to say that this will limit the effectiveness of the second titration? Alternatively, the weaker acid will require less base to neutralise, and so we can use the amount of NaOH used as a determination of the substance? Basically, you're totally on the right track, and the question is very stupid.

[Bubbly_bluey]

Lets hope this isn't another weird question

The answer marked is...

Spoiler

C

[kiwiberry]

Lets hope this isn't another weird question

The answer marked is...

Spoiler

C

I got B! You just times it by its molar mass, right?

[J.B]

Hey! the answer is not D because you would have done esters I believe? So ethly propanoate is an ester that is produced from an alkanol and alkanoic acids.Usually esters produce nice fruity smells that are commonly used for perfumes, cosemetics and flavourings. They are not usually used for dissolving anything (I'm not sure) just mainly to produce smells.
As for the second question, it wouldnt hurt to know one or two just in case. Use as an example for long responses? Generally they wouldn't ask for specific flavouring come from which reactants.
Hope this helps

Ok thank you,
It was just that my teacher told us they are also used as solvents.

[Bubbly_bluey]

I got B! You just times it by its molar mass, right?

I got B as well! Not sure why C is the answer. Could be an error or something :/

[Kekemato_BAP]

I need clarification on this question.. I'm not sure if it is A or D because all acids have hydrogen, but metal salts are also formed

[jakesilove]

I need clarification on this question.. I'm not sure if it is A or D because all acids have hydrogen, but metal salts are also formed

It was my understanding that a metal + acid = salt + hydrogen! So you're totally correct to be confused, A and D are both correct

Stupid question,

[legorgo18]

Hey so this question is in my homework:

A chemist mixes 270mL of 4M zinc nitrate solution with 185 mL of 5M magnesium sulfate solution to form a precipitate. She then ........

so part a is to write a balanced chemical equation to represent the reaction.

My question is, what is the precipitate? All nitrates are soluble, sulfate has no restriction with zinc.... is this an error in my homework? Thanks for answering

[Kekemato_BAP]

Hey so this question is in my homework:

A chemist mixes 270mL of 4M zinc nitrate solution with 185 mL of 5M magnesium sulfate solution to form a precipitate. She then ........

so part a is to write a balanced chemical equation to represent the reaction.

My question is, what is the precipitate? All nitrates are soluble, sulfate has no restriction with zinc.... is this an error in my homework? Thanks for answering

This question is designed to test knowledge on solubility rules. Good job on identifying that both products are soluble! The answer would be "none" since no insoluble precipitate is formed.

[legorgo18]

This question is designed to test knowledge on solubility rules. Good job on identifying that both products are soluble! The answer would be "none" since no insoluble precipitate is formed.

Bruh the funniest thing is the next part says calculate the theoretical mass of precipitate formed...

[Kekemato_BAP]

I need help understanding the different acid/base theories like Lavoisier, Davy, Arrhenius, B/L
I know the main definition but need help on the disadvantages of each theory. Thanks

[RuiAce]

I need help understanding the different acid/base theories like Lavoisier, Davy, Arrhenius, B/L
I know the main definition but need help on the disadvantages of each theory. Thanks

For the acid-base theories, the disadvantages are essentially what is not addressed by each theory.

Lavoisier stated that all acids contain oxygen. Counterexample: HCl.

Davy stated that all acids contain hydrogen. Counterexample: NaHCO₃.

Arrhenius stated that all acids ionise in water to produce H⁺. This was quite an improvement from theories in the past and is quite accurate. He also defined degree of ionisation, as well as taking the concept over to bases as well, claiming they must contain OH^-. This also lead to the concept of neutralisation.
The counterexample to this one is mostly with bases. Again, NaHCO₃ is a counterexample, but for a different reason. It's a base that doesn't contain the hydroxide ion.

As far as things go, modern day acids and bases are always defined based off B/L theory. At least, that's the convention, and how HSC goes. In reality acids and bases are categorised into two types: B/L and Lewis acids/bases. But you do not need to go any further - take it for granted that B/L is the most accurate of the lot. It also has the ability to address the amphiprotic nature of substances as well.

[jakesilove]

I need help understanding the different acid/base theories like Lavoisier, Davy, Arrhenius, B/L
I know the main definition but need help on the disadvantages of each theory. Thanks

Hey! Let's take a look at the scientists in the curriculum.

Lavoisier claimed that acids were substances that contained Oxygen. This is because he noticed that non-metal oxides were acidic in solution. However, this theory was disproven, as many basic substances contain Oxygen (eg. NaOH) and some acids did not contain Oxygen (eg. HCl).

Davy updated the theory, claiming that Acids were substances that contained Hydrogen atoms that could be replaced by metal ions. He claimed this because he notices that, when acids react with metals, the metal seemed to replace Hydrogen, creating Hydrogen gas. However, thus notion only allows acids to be defined in conjunction with metals. Clearly, acids can be acidic without being introduced to a metal species

Arrhenius suggested that acids were substances that ionised in solution to produce Hydrogen ions. This was a really solid theory, as it defined acids independently of any other species. He also defined strong and weak acids! Now, the reason that Arrhenius' theory is wrong gets a bit technical (you can find more information on that here) which is why for most cases his theory is actually fine to use! However, it has been updated.

The B-L theory of acids and bases simplifies everything. Acids are proton donors, and bases are proton acceptors. Easy!

[hanaacdr]

Hi,
just 2 questions
in the Haber process why is monitoring the reaction vessel for level of oxygen critical?

also why does the concentration of hydrogen and nitrogen have to be in a ratio of 1:3 in the Haber process?

thank you

[kiwiberry]

Hi,
just 2 questions
in the Haber process why is monitoring the reaction vessel for level of oxygen critical?

also why does the concentration of hydrogen and nitrogen have to be in a ratio of 1:3 in the Haber process?

thank you

Oxygen reacts with hydrogen explosively under high temperatures, so the ingoing gas stream must be monitored so all oxygen is removed before it enters the reaction vessel! The 1:3 ratio for N:H maximises efficiency of conversion into ammonia, because a buildup of excess gas will take up space in the vessel and also pose a risk of explosion.