How many moles of sodium carbonate soluble are present in 400cm3 of sodium carbonate solution which has a concentration of 0.25 mol/dm3
с (Na₂CO₃) = n(Na₂CO₃)/V(Na₂CO₃ solution);
n (Na₂CO₃) = c (Na₂CO₃) * V (Na₂CO₃ solution);
V = 400 cm3 = 0.4 dm3;
n = 0.25 mol/dm3 * 0.4 dm3 = 0.1 mole;
answer: 0.1 mole.