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Calculation questions

So, I'm struggling with the calculation questions on uniying concepts and trendsand patterns and saw a question worth 15 marks solely on calcualtions. I would have flopped that paper. So I need help with these 2please:

1)A student dissolved 0.270g of an anhydrous compound of iron and chlorine in water. The student then added an excess of aqueous silver nitrate to precipitate aqueous chloride ions as solid silver chloride. The student collected 0.610g AgCl(s). (5 marks)
Calcualte the formula of the iron compound.

2) A student analysed an unsaturated carboxylic acid, A, using a titration procedure. The student dissolved 2.580g of the compound in water and made the solution up to 250cm3. The student titrated 25.0cm3 of this solution with 0.1263moldm-3 NaOH. The volume of NaOH (aq) required to reach the end point was 23.75cm3.
Each molecule of A has one acidic hydrogen atom and A behaves as a monoprotic (or monobasic) acid.
Calculate the molar mass of the unsaturated carboxylic acid.
Determine the molecular formula and possible displayed or skeletal formula of the carboxylic acid.
Reply 1
Moles AgCl = 0.610/ (108+35.5) = 4.25 x 10^-3

Ag+ + Cl- --> AgCl

so one mole of chloride ions produces one mole of precipitate,

so moles of chloride ions = 4.25 x 10^-3

Mass of Cl ions = 4.25 x 10^-3 x 35.5 = 0.15g

so 0.15g of the compound is chlorine, and 0.12g is iron
Calc the emperical formulae:

0.15/35.5 = 4.23x10^-3 mol 0.12/56 = 2.14x10^-3

Ratio Cl to iron is 2:1

So FeCl2 I think

2) Moles NaOH = 23.75/1000 x 0.1263 = 3.0 x 10^-3 moles.

RCOOH + NaOH --> RCOO-Na+ + H2O

Since R is monoprotic, 1mol NaOH reacts with 1 mole of acid to produce 1 mol of water

so moles acid = 3.0 x 10^-3 ( in 25cm^3)

Moles acid in total = 10 x 3.0x10^-3 = 0.030 (to bring up to 250cm^3)

so Mass of acid in total should = 0.030 (moles) x it's RFM

We know the mass which is 2.580g, so we must calculate the RFM

RFM = Mass/Moles = 2.580g/0.030 = 86

Acid is unsaturated so has a C=C bond...

CH(CH3)=CH-COOH
Reply 2
Thank you so much. They are the right answers by the way. You must be really clever!
Reply 3
Do you have any tips as to how to answer those kinds of questions? How did you know what to do?
Reply 4
Umm.. even if you dont understand the chemistry, you should be able to work with the data they've given you.. you always have to calculate the moles at the start, and then work from there to find out what you want.. also you need to recall simple things such as the equations for the ppt formation and acid base equilibria as in this question..

The reason why I found it easy is cuz our teacher makes us do a lot of these in school :p:, so practise more and you'll get the hang of it
Reply 5
where does the 10 comes from in the second question?
Original post by sneha2096
where does the 10 comes from in the second question?

The moles calculated for A was in 25cm^3 so you need to calculate moles in 250cm^3 by multipling it by 10.