Question

Does adding 1 mol of NaCl to 1 kg of water lower the vapor pressure of water to the same extent, a lesser extent, or a greater extent than adding 1 mol of C6H12O6 to 1 kg of water? And why?

Answer 1

Answer:

Adding 1 mol of NaCl to 1 kg of water lower the vapor pressure of water to the same extent  by adding 1 mol of $C_06H_{12}O_6$ to 1 kg of water.

Explanation:

1) Moles of NaCl ,$n_1=1 mol$

Mass of water = m= 1 kg = 1000 g

Moles of water = $n_2=\frac{1000 g}{18 g/mol}=55.55 mol$

Vapor pressure of the solution = $p$

Vapor pressure of the pure solvent that is water = $p_o=17.5 Torr$

Mole fraction of solute(NaCl)= $\chi_1=\frac{n_1}{n_1+n_2}$

$\frac{p_o-p}{p_o}=\frac{n_1}{n_1+n_2}$

$\frac{17.5 Torr-p}{17.5 Torr}=\frac{1 mol}{1 mol+55.55 mol}$

$p=17.19 Torr$

The vapor pressure for the NaCl solution at 17.19 Torr.

2) Moles of sucrose ,$n_1=1mol$

Mass of water = m  = 1 kg = 1000 g

Moles of water = $n_2=\frac{1000 g}{18 g/mol}=55.55 mol$

Vapor pressure of the solution = $p'$

Vapor pressure of the pure solvent that is water = $p_o=17.5 Torr$

Mole fraction of solute ( glucose)= $\chi_1=\frac{n_1}{n_1+n_2}$

$\frac{p_o-p}{p_o}=\frac{n_1}{n_1+n_2}$

$\frac{17.5 Torr-p}{17.5 Torr}=\frac{1 mol}{1 mol+55.55 mol}$

$p'=17.19 Torr$

The vapor pressure for the glucose solution at 17.19 Torr.

p = p' = 17.19 Torr

Adding 1 mol of NaCl to 1 kg of water lower the vapor pressure of water to the same extent  by adding 1 mol of $C_06H_{12}O_6$ to 1 kg of water.

Answer 2

NaCl will cause more lowering of the vapor pressure compared to sugar because NaCl dissolves easily in water.

Data;

• Number of moles = 1mol
• Mass = 1kg

Vapor Pressure

Vapor pressure is a measure of the possibility  of a substance to change into the gaseous or vapor state.

When we lower  the vapor pressure of a substance, it is a colligative property, which depends on the number of the solute particles dissolved in the solvent (water) not regarding the nature of the individual element. Therefore, when we have more of the particle in the solution, the greater the depression(lowering) in the vapor pressure.

NaCl is a strong electrolyte when dissolved in water, it dissociates and produces two ions which sodium and chloride ions.

$NaCl\to Na^+ + Cl^-$

while C6H12O6 is a molecular compound which is a non-electrolyte and does not dissociate in the solution.

Since NaCl generates more solute particle in the solution, it will definitely cause lowering in the vapor pressure to a greater extent.

Learn more on vapor pressure here;

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