HSC Chemistry Question Thread

[Kate Science]

Hi I'm not sure how to work out the heat produced when 1.0L of ethanol undergoes complete combustion, given the molar heat of combustion as 1370KJ/mol and its density 0.80g/mL
Thanks

Hiya,
So here are the steps:

1. 1.0L of ethanol = 0.8 g/mL * 1000mL = 800g ethanol burned (mass)

2. mol ethanol burned = (mass)/(molar mass ethanol) = X

3. heat produced = (molar heat)*(mol) = 1370KJ/mol * X = answer in kilojoules

You should be able to fill in the numbers okay from there.

If that's not clear, just chuck a reply and I can help you fill it in further

Cheers

[Mate2425]

Hey, quick question, does any one know of any good Websites/ resources to understand titration calculations, i am finding it hard to understand the concept/ formulative way of approaching these sorts of question.
Thank you.

[mxrylyn]

Hello

Im doing some practice papers for the option "Shipwrecks and Corrosion" and im having a lot of trouble doing questions where I have to relate or compare things that I didn't lern in tademn at school such as


"Identify the ions normally dissolved in the ocean and relate these findings to the corrosion of metals.

"Explain why the concentrations af selected gasses chnages with increasing depth in the ocean and relate these findings to the corrosion of metals at different depths"

"Identify the effects of changes in temperature with depth on the rate of corrosion of metals" and

"Identify the chemical reactions involved in corrosion of metals at great depth and explain how these reactions differ from corrosion in shallow water."

Does anyone have an idea as the correct answer to these questions as well as tips on how to answer questions like this 

Thank you

[clovvy]

Hey, quick question, does any one know of any good Websites/ resources to understand titration calculations, i am finding it hard to understand the concept/ formulative way of approaching these sorts of question.
Thank you.

Titration is just one of those topics where you just need to be strong with your chem skills(interpreting data and info given) and basics on moles calculations like limiting reagents, stoic ratios from yr 11.... Titration is basically calculating amounts of acid or bases needed to neutralise a solution (like 100mL of 0.1 mols of HCL, how much 0.05 mol of NaOH needed to neutralise it).... The best way to study for these type of stuff is just practice questions.... If you need help with questions just ask here, either me or someone stronger than me may be able to answer them (or at least try to guide you through them)....

[Mate2425]

Titration is just one of those topics where you just need to be strong with your chem skills(interpreting data and info given) and basics on moles calculations like limiting reagents, stoic ratios from yr 11.... Titration is basically calculating amounts of acid or bases needed to neutralise a solution (like 100mL of 0.1 mols of HCL, how much 0.05 mol of NaOH needed to neutralise it).... The best way to study for these type of stuff is just practice questions.... If you need help with questions just ask here, either me or someone stronger than me may be able to answer them (or at least try to guide you through them)....

Thank you, much appreciated!

[varun.amin]

Hey, quick question, does any one know of any good Websites/ resources to understand titration calculations, i am finding it hard to understand the concept/ formulative way of approaching these sorts of question.
Thank you.

Hi Mate2425,

Usually I like to structure titration calculations using a set of steps to find the answer. Just be careful using this method based on what the question is asking. 

1. Write out a balanced equation from the given information (this helps later with molar ratios)
2. Write out the following, usually one of the concentrations will be missing and is usually what you need to find:
Volume(base)=      Concentration(base)=           Volume(acid)=         Concentration(acid)=
3. Calculate the moles of the known acid/base (this will be the one where you are given both the concentration and the volume using the formula n=cv)
4. See if you need to adjust the moles of the unknown concentration of acid/base based on the molar ratio equation above (say for example you might have twice as many moles of the unknown acid/base, at this stage you would have to double the known moles of the acid/base, to get the moles of the unknown acid/base)
5. Then finally you need to calculate the concentration of the unknown substance using the formula c=n/v where we know both n as the amount of moles and v as the volume

Hope this helps!

[cnimm2000]

Hey guys,
Why do we need to do sequencing when testing cations in mixtures
it would be great if you could help!

[RuiAce]

Hey guys,
Why do we need to do sequencing when testing cations in mixtures
it would be great if you could help!

Don't remember the flow chart off the top of my head anymore but the main culprit is the solubility rules.

Thanks to solubility rules, cation tests actually exist. But unfortunately, some anions will precipitate with more than one cation. (A big one is the carbonate ion, which precipitates with almost everything; only exclusions being ammonium and group I metals as far as I know of.) We want to design our cation tests so that a particular sequence of steps can only lead to one possible cation being what's present, as opposed to multiple.

[earlgrey1803]

Hey guys - was wondering if I could grab some help on a titration question I've been struggling with - whenever I do it I always seem to end up with 1.72 grams of calcium carbonate which doesn't quite work...

"A student wanted to determine the percentage of calcium carbonate in sticks of coloured chalk.
1.    A sample of coloured chalk weighing 0.235g was reacted with 40.00mL of 1.00 mol L-1 HCl (excess).
2.   After the reaction was complete the solution was transferred to a 250mL volumetric flask and filled up to the mark with distilled water.
3.   25.00mL portions of this solution were then titrated against 0.150 mol L-1 sodium hydroxide using methyl orange as the indicator.

Calculate the percentage purity of the chalk."

The average amount of NaOH used is  23.96mL

Any help would be greatly appreciated!!

[emily_p]

Hey guys - was wondering if I could grab some help on a titration question I've been struggling with - whenever I do it I always seem to end up with 1.72 grams of calcium carbonate which doesn't quite work...

"A student wanted to determine the percentage of calcium carbonate in sticks of coloured chalk.
1.    A sample of coloured chalk weighing 0.235g was reacted with 40.00mL of 1.00 mol L-1 HCl (excess).
2.   After the reaction was complete the solution was transferred to a 250mL volumetric flask and filled up to the mark with distilled water.
3.   25.00mL portions of this solution were then titrated against 0.150 mol L-1 sodium hydroxide using methyl orange as the indicator.

Calculate the percentage purity of the chalk."

The average amount of NaOH used is  23.96mL

Any help would be greatly appreciated!!

Hello!! I’ve attached an image of calculations I’ve gone through. Since the question is a back titration these are some of the steps I’ve gone through:

1. Calculate moles of NaOH reacted
2. Calculate moles of HCl reacted
3. Calculate moles of HCl in beaker (25ml —> 250ml)
4. Calculate moles of HCl used in reaction with chalk from initial - excess
5. Calculate moles of CaCO3 used, calculate mass and then find percentage

The percentage I got was 86.5% to 3 sf, if you have the answer could you check with this and confirm?

Hope this helps! Let me know if I’ve stuffed anything up or if something doesn’t make sense

[key to success]

Hey, sorry for intervening, the question looked good and so did the solution so thought i'll check it out!
Just in your working can explain the step where the work out the number of moles of the HCL in excess- what formula did you use and what does it even mean by 'in excess'?
Thanks!

[emily_p]

Hey, sorry for intervening, the question looked good and so did the solution so thought i'll check it out!
Just in your working can explain the step where the work out the number of moles of the HCL in excess- what formula did you use and what does it even mean by 'in excess'?
Thanks!

Definitely! By HCl(excess) I was referring to the total remaining HCl after the reaction - ie all the moles of HCl in the beaker, probably should have made this clearer whoops! Since only a 25 ml sample was taken from a 250 ml beaker, I multiplied the moles in the sample by 10 to get the moles in the beaker.

Alternatively, you could use C=n/V twice:

C(titrated HCl) = 0.003594/0.025
   = 0.14376 mol/litre

C(beaker) = n/0.25 ==> sub in C of titrated HCl (conc of sample will be same as conc of whole thing)
0.14376= n/0.25
n(beaker) = 0.14376 x 0.25
   = 0.03594

So the answer would be the same as multiplying by 10 but in case you're a bit unsure of it C=n/V works too!!

[earlgrey1803]

Definitely! By HCl(excess) I was referring to the total remaining HCl after the reaction - ie all the moles of HCl in the beaker, probably should have made this clearer whoops! Since only a 25 ml sample was taken from a 250 ml beaker, I multiplied the moles in the sample by 10 to get the moles in the beaker.

Alternatively, you could use C=n/V twice:

C(titrated HCl) = 0.003594/0.025
   = 0.14376 mol/litre

C(beaker) = n/0.25 ==> sub in C of titrated HCl (conc of sample will be same as conc of whole thing)
0.14376= n/0.25
n(beaker) = 0.14376 x 0.25
   = 0.03594

So the answer would be the same as multiplying by 10 but in case you're a bit unsure of it C=n/V works too!!

Thanks for all your help! The answer looks right but I don't have the answers so I can't confirm...
All my working matched up with yours except for this step which I completely forgot which is why I was getting a wildly incorrect answer!

Thanks 

[sarra.mourad2000]

This question
"Explain why alkanes and corresponding alkenes have similar properties, but very different chemical properties" comes up all the time but I can never get around to figuring out how to answer it... HELPPP 

[Ace2018]

This question
"Explain why alkanes and corresponding alkenes have similar properties, but very different chemical properties" comes up all the time but I can never get around to figuring out how to answer it... HELPPP 

I presume you meant "Explain why alkanes and corresponding alkenes have similar PHYSICAL properties, but very different chemical properties" :

When explaining physical properties, you would talk about the intermolecular bonds i.e. the bonds BETWEEN molecules. So, we know that both alkanes and alkenes can only form weak dispersion forces with other molecules, therefore they both are insoluble in water, have a low boiling point and low melting point.

Whereas with explaining chemical properties, you would talk about the intramolecular bonds, i.e. the bonds WITHIN molecules. So, we know that alkanes have single bonds whilst alkenes have double bonds. This means that the alkenes' double is highly reactive and can readily undergo addition reaction.

I hope that help.