Chemistry calculations (volumetric analysis calculations) help?

Question

3.50 cm^3 of 1.00 mol/dm^3 nitric acid reacts with 25.0 cm^3 of sodium carbonate solution. calculate the concentration of sodium carbonate solution in mol/dm^3

Answer 1

3.50 cm^3 = 0.0035 dm^3
Moles HNO3 = Molarity x Volume = 1.00 mol / dm^3 x 0.0035 = 0.0035
The reaction is :
2 HNO3 + Na2CO3 >> 2 NaNO3 + H2CO3
the ratio between HNO3 and Na2CO3 is 2 : 1
2 : 1 = 0.0035 : x
x = 0.00175 moles Na2CO3
25 cm^3 = 0.025 dm^3
[Na2CO3] = 0.00175 / 0.025 =0.070 mol/dm^3

Answer 2

3.5 x 1/1000 x 1/2 x 1000/25 mol/dm3