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Calculating the concentration - Chemistry Moles question?

I understand question 1 but I am struggling to result in the answer 0.13moldm³- for question 2. Please can you help?

1. Calculate the mass of anhydrous sodium carbonate, Na₂CO₃, needed to make exactly 500 cm³ of aqueous sodium carbonate of concentration 0.050 mol dm³-.

Molar mass for Na₂CO₃ = 106
106 × 0.050 = 5.3 for 1,000
5.3 ÷ 2 = 2.65 g

2. 3.45 g of anhydrous sodium carbonate, Na₂CO₃, was dissolved in water and the solution made up to 250 cm³ in a volumetric flask. Calculate the concentration of the solution in mol dm³-.

106 in 1000
26.5 in 250
0.13moldm³-.

Thank you so so much!

Reply 1

mamasaywhatnow

Na2CO3 - Mr = 106, mass = 3.45, therefore moles = 0.0325 (3sf)
concentration = mass or moles / volume
concentration = 0.0325 / 0.25 = 0.13 mol/dm3

volume is measured in dm3, so you have to convert the cm3 to dm3 (divide by 1000)