Question

determine the mass of potassium carbonate which must be dissolved in distilled water to make 250cm of potassium carbonate solution with a concentration of 0.2 mol dm3

Answer 1

Task:

determine the mass of potassium carbonate which must be dissolved in distilled water to make 250cm of potassium carbonate solution with a concentration of 0.2 mol dm3

Solution:

The molarity of solution is

$$C (M) = n (mol) / V (L)$$

C – molarity (M)

n – number of moles of substance

V - volume of solution (L)

First of all we have to convert mol/dm³ to mol/L

$$1 \mathrm{dm}^3 = 1 \mathrm{L}$$ $$C \left(K _ {2} C O _ {3}\right) = 0.2 \mathrm{mol}/\mathrm{dm}^3 = 0.2 \mathrm{mol}/\mathrm{L} = 0.2 \mathrm{mol}/\mathrm{L}$$

Also we have to convert volume in cm3 to L

$$1 \mathrm{cm}^3 = 1 \mathrm{mL} = 1 \cdot 10^{-3} \mathrm{L}$$

V(solution) = 250 mL = 250 · 10⁻³ L = 0.250 L

The number of moles of K₂CO₃ is

$$n (mol) = C (M) \cdot V (L)$$ $$n \left(K _ {2} C O _ {3}\right) = 0.2 \cdot 0.250 = 0.05 \mathrm{mol}$$

The mass of K₂CO₃ is

$$m (g) = n (mol) \cdot MW (g/mol)$$

The molar weight consists of atomic weights of elements taken from the periodic table

$$MW \left(K _ {2} CO _ {3}\right) = 2 \cdot MW (K) + MW (C) + 3 \cdot MW (O) = 2 \cdot 39 + 12 + 3 \cdot 16 = 138 \mathrm{g/mol}$$ $$m \left(K _ {2} C O _ {3}\right) = 0.05 \cdot 138 = 6.9 \mathrm{g}$$

Answer: $m(K_2CO_3) = 6.9 \mathrm{g}$