Question

What is the freezing point of a solution of 7.15 g MgCl2 in 100 g of water? K f for water is 1.86°C/m. What is the freezing point of a solution of 7.15 g MgCl2 in 100 g of water? K f for water is 1.86°C/m. -0.140°C -2.80°C -1.40°C -4.18°C

Answer 1

THE NEW FREEZING POINT IS -4.196 °C

Step-by-step explanation:

ΔTf = 1 Kf m

molarity of MgCl2:

Molar mass = (24 + 35.5 *2) g/mol

molar mass = 95 g/mol

7.15 g of MgCl2 in 100 g of water

7.15 g = 100 g

(7.15 * 100 / 1000) = 1000 g or 1 L or 1 dm3

= 0.715 g /dm3

Molarity in mol/dm3 = molarity in g/dm3 / molar mass

= 0.715 g /dm3 / 95 g/mol

m = 0.00752 mol/ dm3

So therefore:

ΔTf = i Kf m

1 = 3 (1 Mg and 2 Cl)

Kf = 1.86 °C/m

M = 0.752 moles

So we have:

ΔTf = 3 * 1.86 * 0.752

ΔTf = 4.196 °C

The new freezing point therefore will be 0 °C - 4.196 °C which is equals to - 4.196 °C