### Higher cheemistry 2011 calculation help!!

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• #### Message 1.

Posted by U15228749 (U15228749) on Tuesday, 14th May 2013

hi im stuck on these type of calculation in the multichoice section how is the answer D?

A mixture of potassium chloride and
potassium carbonate is known to contain
0·1 mol of chloride ions and 0·1 mol of
carbonate ions.
How many moles of potassium ions are
present?
A 0·15
B 0·20
C 0·25
D 0·30

how would i do this, help much appreciated!

• #### Message 2

, in reply to message 1.

Posted by MissChemTeacher_Bitesize (U15172316) on Wednesday, 15th May 2013

Hi There!

I understand this type of question can be a bit tricky. The key to answering this is to use your valency cross over method to write the formula for the two compounds.

potassium chloride KCl
potassium carbonate K₂CO₃

You then need to look at the proportion (ratio) of each portion of your compound.

potassium chloride KCl has one K for every Cl
potassium carbonate K₂CO₃ has two K for every CO₃

So in the KCl as there are 0.1 moles of Cl ions there must be 0.1 moles of K ions.

In the K₂CO₃ there are 0.1 moles of CO₃ ions but we'd need to double that to get the K ions as there are double the number of K ions in the original formula. So we'd have 0.2 moles of K ions.

In total we have 0.1 moles of K ions from the KCl and 0.2 moles of K ions from the K₂CO₃ which would equal 0.3 moles altogether.

Hope you get this and it helps you out.

The 2001 paper Q4 and 2008 paper Q4 have similar questions if you want to test yourself and check you've got the hang of it.

Good Luck with the revision.
Cheers Miss_Chem

• #### Message 3

, in reply to message 2.

Posted by U15228749 (U15228749) on Wednesday, 15th May 2013

thankyou, and how would i do this one, im rubbish at multi choice :L lol

Which of the following compounds does not
have isomeric structures?
A C2HCl3
B C2H4Cl2
C Propene
D Propan-1-ol

its from 2009/ higher

• #### Message 4

, in reply to message 3.

Posted by MissChemTeacher_Bitesize (U15172316) on Thursday, 16th May 2013

Hey!

I've had a wee look at this question and it's gonna be a bit tricky to explain without being able to draw the structures. I'll try my best - here goes...

A: C₂HCl₃ You can't get an isomer of this compound. Two carbons are joined together with a C=C. One carbon will have a H and a Cl, the other will have two Cl's attached. If you flip this round it'll still be the same structure.

B: C₂H₄Cl₂ There are two possible isomers of this compound.
Option 1: The carbons are joined with a single bond C-C. One of the carbons will have 3H's attached and the other will have one H and two Cl's. This structure is called 1,1-dichloroethane.
Option 2: The carbons are joined with a single bond C-C. One carbon will have 2H's and one Cl. The other carbon will have 2H's and one Cl. This structure is called 1,2-dichloroethane.

C: propene C₃H₆ There are two possible isomers of this compound.
Option 1: normal propene with a C=C.
Option 2: cyclopropane

D: Propan-1-ol
Option 1: normal propan-1-ol with the hydroxyl group (-OH) at the end of the chain.
Option 2: propan-2-ol with the hydroxyl group (-OH) on the second carbon in the chain.

I hope this helps you out and that you understand what I'm on about. Try drawing the structures as I've described them so hopefully you'll have a visual of what I mean.

If you need more of an explanation get back in touch.

Cheers
Miss_Chem

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