HSC Chemistry Question Thread

[beau77bro]

where can i find some really good chem calculation questions?

[itssona]

Hey would anyone be able to explain this:

Metals high up in the reactivity series (such as sodium, calcium and magnesium) have carbonates that need a lot of energy to decompose them.

?

If metals that are more reactive need more energy to decompose, then that means they have stronger bonds..but how?

Thank you

[jakesilove]

Hey would anyone be able to explain this:

Metals high up in the reactivity series (such as sodium, calcium and magnesium) have carbonates that need a lot of energy to decompose them.

?

If metals that are more reactive need more energy to decompose, then that means they have stronger bonds..but how?

Thank you

Hey! You've confused the ELEMENT, and the associated COMPOUND.

Sodium is very, very reactive. However, NaCl (table salt) is very unreactive! This makes sense; since Sodium is so react, it will try really, really hard to stay as an unreactive compound, rather than decompose into its constituent elements. So, a lot of energy is required to force the stable Sodium to become unstable (ie. to go from a compound back to an element).

Does that kind of make sense? A lot of energy is required to break intermolecular forces in a compound containing reactive elements, BECAUSE the element is reactive and wants to stay bonded!

[itssona]

Hey! You've confused the ELEMENT, and the associated COMPOUND.

Sodium is very, very reactive. However, NaCl (table salt) is very unreactive! This makes sense; since Sodium is so react, it will try really, really hard to stay as an unreactive compound, rather than decompose into its constituent elements. So, a lot of energy is required to force the stable Sodium to become unstable (ie. to go from a compound back to an element).

Does that kind of make sense? A lot of energy is required to break intermolecular forces in a compound containing reactive elements, BECAUSE the element is reactive and wants to stay bonded!

OMFG THANK YOU SO MUCH DJFSISK

[jakesilove]

OMFG THANK YOU SO MUCH DJFSISK

No worries; totally understand why you thought what you though!

[anotherworld2b]

Hi i attempted to do these two questions but I'm having some trouble

[MisterNeo]

Hi i attempted to do these two questions but I'm having some trouble

Hi!! I see what you did wrong in Q15. The equation you wrote was correct but your calculations didn't take into account that NaOH was 2 moles for each H2SO4. Thus, your calculations needed to divide moles of NaOH by 2.
I have attached my calculations for Q16. Hope this helps!! Feel free to ask for any clarification..

[ellipse]

Would someone please help me with a titration doubt please

So in most of the titration methods i read, they wash the conical flask with distill water, but they don't dry it or something. Isn't this inaccurate? Because when we transfer 25 ml of acid (or base) into it, the volume will increase (due to the left over distill water) but the moles of the acid (or base) will be the same, thus leading to a lower concentration of the acid (or base) than what was intended. So when we calculate the concentration, we will get a different answer than what was intended.

Could someone clarify this please

[jakesilove]

Would someone please help me with a titration doubt please

So in most of the titration methods i read, they wash the conical flask with distill water, but they don't dry it or something. Isn't this inaccurate? Because when we transfer 25 ml of acid (or base) into it, the volume will increase (due to the left over distill water) but the moles of the acid (or base) will be the same, thus leading to a lower concentration of the acid (or base) than what was intended. So when we calculate the concentration, we will get a different answer than what was intended.

Could someone clarify this please

Hey!

Once we've transferred the solution to the conical flask, the concentration no longer matters. The important thing is the number of moles in the flask for the Base to neutralise. Again, the concentration is totally irrelevant at this stage. We need to figure out the number of moles required of base to neutralise the number of moles of acid present in the flask. For this, we use the chemical formula etc. and carry out the usual steps.

I could pour as much water (if it's pH was exactly 7) into the conical flask as I want! The amount of base required to neutralise it would be the same.

[jakesilove]

Hey!

Once we've transferred the solution to the conical flask, the concentration no longer matters. The important thing is the number of moles in the flask for the Base to neutralise. Again, the concentration is totally irrelevant at this stage. We need to figure out the number of moles required of base to neutralise the number of moles of acid present in the flask. For this, we use the chemical formula etc. and carry out the usual steps.

I could pour as much water (if it's pH was exactly 7) into the conical flask as I want! The amount of base required to neutralise it would be the same.

I don't think I explained myself very well

Let me know if you need me to clarify!

[anotherworld2b]

Hi
I was just wondering
We are required to write a step by step method, which allows someone to determine the total acidity of wine. Expressing the final determination of total acidity of the wine in grams per litre (g.L-1) Include the proposed calculations in this plain, using x and multiples of x to represent unknown value/s. Assume the main and only contribution of the acidic properties of wine is tartaric acid.

I rewrote the method. I was wondering if someone could have a look at it please?
I also tried to the calculations which will determine tartaric acid concentration in wine but I'm not sure how to find the total acidity of the wine in grams per litre.
 

Hey!

So, we're titrating Tartaric acid and Sodium hydroxide. We have the concentration (and thus moles) of Sodium hydroxide; and we can find the volume (and thus concentration) of Tartaric acid using the titration process. First, we need a chemical equation.



I had to google this. From here, we can do straight forward titration calculations to find the concentration of acid. Find the number of moles of NaOH used, and halve it to get the number of moles of Tartaric acid. Finally, divide the moles of acid by the volume used to get the concentration.

[bsdfjnlkasn]

Hey there! I sort of guessed my way through this question but can't find any solid answers online, all there is are marking guidelines but I'm not finding them too helpful because I'm stuck. If someone could check my part a) and explain why it's those two solutions as well as an approach for q17 I'd really appreciate it. Thank you!!

[RuiAce]

Hey there! I sort of guessed my way through this question but can't find any solid answers online, all there is are marking guidelines but I'm not finding them too helpful because I'm stuck. If someone could check my part a) and explain why it's those two solutions as well as an approach for q17 I'd really appreciate it. Thank you!!


(Image removed from quote.)

I actually disagree with the answers. I think the reaction between solutions 1 and 2 is this one

[anotherworld2b]

Could i have some help with this question please?

[anotherworld2b]

I see now thank you for your help
I was trying to do this question. But i havent dealt with density before so I'm not sure how to start.

This is how I did it. Find moles of HCl used, then minus the excess by finding moles of NaOH used to neutralise. Use that final HCl used to find the moles of CaCO3 reacted. This ends up being 0.00975mol. Multiply that by the MM of CaCO3 to get 97.5%, which is divided by 1g of calcite which is also 97.5%, and it rounds up to 98% like the answer says.
Hope this makes sense. Feel free to ask for clarification