Question

500 ml of a solution contains 12.6 g of oxalic acid (Mol wt = 126). 10 ml of this solution is diluted to 100 ml in a flask. What is molarity of the resulting solution?

a) 0.2
b) 0.1
c) 0.02
d) 0.01

Answer 1

12.6 gm = 12.6/126 moles = 0.1 moles  of Oxalic acid in 500 ml.
So    0.1 / 50 = 0.002 moles of  Oxalic acid in 10 ml of solution

now 90 ml of water is added.  
Concentration:  0.002 moles / 100 ml  = 0.02 Molar.

Answer 2

Since dilution takes place..hence no.of moles will be same
So m1v1=m2v2
M1 = 12.6/126 *2. (1000/500) in litres
We hav to find m2
0.2 * 10 =100 m2
M2 = 0.02 M
Hope it helps