# Exam question

1. Jun 2, 2009

### mouk92

(a) An acid, HX, reacts with sodium hydroxide as shown in the equation
below:

H2X(aq) + 2NaOH(aq) -----> 2Na+(aq) + X2-(aq) + 2H2O

A solution of this acid was prepared by dissolving 1.92 g of H2X in water and making the volume up to 250 cm3 in a volumetric flask.

A 25.0 cm3 sample of this solution required 21.70 cm3 of 0.150 moldm-3 aqueous NaOH for complete reaction.

(i) Calculate the number of moles of NaOH in 21.70 cm3 of 0.150 moldm-3 aqeous NaOH.

(ii) Calculate the number of moles of H2X which reacted with this amount of NaOH. Hence, deduce the number of moles of H2X in the 1.92 g sample.

Moles of H2X in 25.0 cm3 of solution .

Moles of H2X in 1.92 g sample

(iii) Calculate the relative molecular mass, Mr, of H2X.

3. The attempt at a solution[/b

part i) is 21.7 × 10-3 × 0.150 = 3.255 × 10-3 (mol)
ii) is In 25 cm 3 = (3.255 × 10-3)/2 = 1.63 × 10-3 (mol)

but i dont know how to get the second part, however i know the answer is 1.63 × 10-2

iii) the anwer is 118, and again how do i get to it

thanks.
1. The problem statement, all variables and given/known data

2. Relevant equations

3. The attempt at a solution
1. The problem statement, all variables and given/known data

2. Relevant equations

3. The attempt at a solution