HSC Chemistry Question Thread

[jakesilove]

could someone please explain the difference between endpoint and equivalence point in a titration?

Hey! See my response to a previous forum question here! Let me know if you need further clarification

[Sssssrr]

Hey! See my response to a previous forum question here! Let me know if you need further clarification

Hey, that did help, but i'm still slightly unsure. To clarify, equivalence point is when the number of moles of the acid equals he number of moles of the base. But then, what its the defitnion of the endpoint, how are they different?
Thanks

[anotherworld2b]

Hi could i please get help with q4, 5 and q9b?

[ml125]

Hi could i please get help with q4, 5 and q9b?

4. Here, you would first start by determining the moles of CO₂ used in the reaction. As the molar ratio of each of the components is 1:1, this will equal the moles of limestone used in the reaction. You can then calculate a percentage using the mass of limestone used and the given value.

5. This is similar to Q4 in that you first need to find a molar value given the information you have – in this case, the moles of H₂. You then follow the same process as the earlier question.

9. As I assume you have already done part a, you should have a molar value for any of the compounds within the equation. The ratio between each of the components is 1:1, therefore you can find the mass of CO₂ from the moles.

Another way you could do this is that from part a, you would have had to find the initial mass of limestone used. From this, you can subtract the answer you get from part a (mass of quicklime). This will be the mass of CO₂. Mass of reactants = mass of products.

[anniez]

Hi everyone!

Could someone pls help me with the attached question? I thought the answer would be D but online it says A and I was like 'what?'. If the answer is A could you pls help explain why? Thanks first!

[RuiAce]

Hi everyone!

Could someone pls help me with the attached question? I thought the answer would be D but online it says A and I was like 'what?'. If the answer is A could you pls help explain why? Thanks first!

D is wrong because the equation isn't balanced

Incomplete combustion produces C, CO, or both.

[anniez]

Ahh okay thanks Rui!

[anotherworld2b]

Thank you very much for your help
I got the wrong answer still for q9b but i am not sure why

I was also wondering how to do these other questions

4. Here, you would first start by determining the moles of CO₂ used in the reaction. As the molar ratio of each of the components is 1:1, this will equal the moles of limestone used in the reaction. You can then calculate a percentage using the mass of limestone used and the given value.

5. This is similar to Q4 in that you first need to find a molar value given the information you have – in this case, the moles of H₂. You then follow the same process as the earlier question.

9. As I assume you have already done part a, you should have a molar value for any of the compounds within the equation. The ratio between each of the components is 1:1, therefore you can find the mass of CO₂ from the moles.

Another way you could do this is that from part a, you would have had to find the initial mass of limestone used. From this, you can subtract the answer you get from part a (mass of quicklime). This will be the mass of CO₂. Mass of reactants = mass of products.

[anotherworld2b]

I have asked about q9 before but iam still confused how to do it as well as this quesiton

[ml125]

Thank you very much for your help
I got the wrong answer still for q9b but i am not sure why

I was also wondering how to do these other questions

In your working you used the wrong molar mass for carbon - it should be 12.01 instead of 10 ^^ The rest of your working is correct, though.

Q5. At STP, one mole of any gas takes up 24.79L. Therefore, if you divide the current volume of gas by this number, you will get the moles of gas. As you have a value for moles and mass, you can find molar mass.

Q9. As Rui stated earlier, this is a reaction between calcium hydroxide (Ca(OH)₂) and CO₂. The solution turns cloudy due to the formation of insoluble calcium carbonate (CaCO₃). Try writing out the equation with the information you have now - see what components are missing from the reactants side and you should be able to determine what the other product is!

I have asked about q9 before but iam still confused how to do it as well as this quesiton

As sodium is a very reactive metal, the reaction it undergoes with water is rapid.The reaction is exothermic, and as a result the sodium metal heats up and burns with an orange flame. One of the products of this reaction is a very commonly used base. The other is a gas.

[RuiAce]

Thank you very much for your help
I got the wrong answer still for q9b but i am not sure why

I was also wondering how to do these other questions

For the ionic equation, make sure you know what exactly an ionic equation is first. And read off the complete balanced equation to get an ionic equation. I know you have a complete equation because you asked for it previously.

Complete equation: Ca(OH)2 + CO2 -> CaCO3 + H2O
(I specifically asked you to figure out the water by yourself for a reason. You need to get used to identifying what's MISSING in equations. The CaCO3 was not obvious, which is why I gave it away, but this one should've been obvious given the hint.)

Full ionic equation: Ca(2+) + 2 OH(-) + CO2 -> Ca(2+) + CO3(2-) + H2O
You need to realise that Ca(OH)2 and CaCO3 are ionic compounds, thus for an ionic equation they can be broken up

Now write down the net ionic equation the question asks you to.

Also, sodium in the water is a very famous experiment. You should well be aware of the immensely high reactivity of group I metals such as sodium.

[MysteryMarker]

Hey guys, just a basic question on acids. (Ha the irony)

There are two beakers, one labelled A and one labelled B. Each beaker has 200ml of either a weak acid or strong acid. The concentration of each acid is unknown. What procedure would you use to determine which beaker contains the strong acid and which contains the weak acid?

Thanks guys.

[conic curve]

Hey guys, just a basic question on acids. (Ha the irony)

There are two beakers, one labelled A and one labelled B. Each beaker has 200ml of either a weak acid or strong acid. The concentration of each acid is unknown. What procedure would you use to determine which beaker contains the strong acid and which contains the weak acid?

Thanks guys.

I believe it's dilution

[anotherworld2b]

Thank you RuiAce and ml125 for your help 

I was wondering how would you know the expected observations from a reaction? I know general base and acid reactions

[RuiAce]

Hey guys, just a basic question on acids. (Ha the irony)

There are two beakers, one labelled A and one labelled B. Each beaker has 200ml of either a weak acid or strong acid. The concentration of each acid is unknown. What procedure would you use to determine which beaker contains the strong acid and which contains the weak acid?

Thanks guys.

I find it quite unfair that they'd ask you something like that in the HSC. I do not know of any practical in the syllabus dot points that relate to this one.

Recommendation: First, perform a titration to determine concentrations.

IF: Concentrations are around the same - Use an indicator (or a pH probe) to figure it out.

IF: Concentrations are noticeably different

React the substances with a weak base. I suspect that something will happen to distinguish between the two. Best let Jake take over here though.

I believe it's dilution

How does dilution tell you anything about whether an acid is weak or strong?