Calculate Concentration - 2

Often you know the mass of a solid dissolved, not the number of moles. You then combine two equations:amount (mol) = mass (g) ÷ M_rconcentration (mol dm^-3) = amount (mol) ÷ volume (dm³)Worked example 1 (mass to concentration): 2.65 g of anhydrous sodium carbonate, Na₂CO₃ (M_r = 106.0), is dissolved and made up to 250 cm³ of solution. What is the concentration?amount = 2.65 ÷ 106.0 = 0.0250 molconcentration = 0.0250 ÷ 0.250 = 0.100 mol dm^-3Worked example 2 (concentration to mass): what mass of sodium chloride (M_r = 58.5) is needed to make 500 cm³ of 0.200 mol dm^-3 solution?amount = 0.200 × 0.500 = 0.100 molmass = 0.100 × 58.5 = 5.85 gThis second type of calculation is exactly what you do before preparing a solution of a chosen concentration: work out the mass, weigh it accurately, dissolve it and make the solution up to the required volume.Keep the working in moles until the last step, and only round the final answer (usually to 3 significant figures).