Question

You want to make up a 0,100 mol.dm^-3 sodium carbonate solution. You have a 200ml volumetric flask and a bottle of sodium carbonate decahydrate (Na2CO2 •10H2O). Calculate the mass of crystal that you need to weigh off. (5,72)

Answer 1

To calculate the mass of sodium carbonate decahydrate (Na2CO3 · 10H2O) needed, we need to use the molar mass and the given volume.

First, let's calculate the molar mass:

2 sodium atoms (2 Na) = 2(22.99 g/mol) = 45.98 g/mol
1 carbon atom (C) = 1(12.01 g/mol) = 12.01 g/mol
3 oxygen atoms (3 O) = 3(16.00 g/mol) = 48.00 g/mol
10 water molecules (10 H2O) = 10(2(1.01 g/mol) + 16.00 g/mol) = 10(18.02 g/mol) = 180.20 g/mol

Now, let's calculate the molar mass of sodium carbonate decahydrate:
Molar mass of Na2CO3 · 10H2O = 45.98 g/mol + 12.01 g/mol + 48.00 g/mol + 180.20 g/mol = 286.19 g/mol

To calculate the mass needed for a 0.100 mol/dm³ solution in a 200 mL volumetric flask, we can use the formula:

Mass (g) = Molar concentration (mol/dm³) × Volume (dm³) × Molar mass (g/mol)

Since the volume is given in milliliters (mL), we need to convert it to decimeters cubed (dm³):

Volume (dm³) = 200 mL ÷ 1000 mL/dm³ = 0.2 dm³

Substituting the values into the formula:

Mass (g) = 0.100 mol/dm³ × 0.2 dm³ × 286.19 g/mol = 5.722 g

Therefore, you need to weigh off approximately 5.72 grams of sodium carbonate decahydrate for the 0.100 mol/dm³ solution in the 200 mL volumetric flask.