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9 moles of Ammonia (NH3) are added to 50 L of H2O at a temperature of 29°C. The vapor pressure of water alone is 29.96 mmHg at 29°C. What is the vapor pressure of the Ammonia solution?

Sagot :

Answer:  The vapor pressure of the solution at [tex]29^0C[/tex] is 29.86 mm Hg

Explanation:

As the relative lowering of vapor pressure is directly proportional to the amount of dissolved solute.

The formula for relative lowering of vapor pressure will be,

[tex]\frac{p^o-p_s}{p^o}=i\times x_2[/tex]

where,

[tex]\frac{p^o-p_s}{p^o}[/tex]= relative lowering in vapor pressure

i = Van'T Hoff factor = 1 (for non electrolytes)

[tex]x_2[/tex] = mole fraction of solute  

=[tex]\frac{\text {moles of solute}}{\text {total moles}}[/tex]

Given : 9 moles of [tex]NH_3[/tex] are dissolved in 50 L or 50000 ml of water

mass of water = [tex]density\times volume = 1g/ml\times 50000ml=50000g[/tex]  

moles of solvent (water) = [tex]\frac{\text{Given mass}}{\text {Molar mass}}=\frac{50000g}{18g/mol}=2778moles[/tex]

Total moles = moles of solute  + moles of solvent = 9 mol + 2778 mol = 2787  

[tex]x_2[/tex] = mole fraction of solute  

=[tex]\frac{9}{2787}=3.2\times 10^{-3}[/tex]

[tex]\frac{29.96-p_s}{29.96}=1\times 3.2\times 10^{-3}[/tex]

[tex]p_s=29.86mmHg[/tex]

Thus the vapor pressure of the solution at [tex]29^0C[/tex] is 29.86 mm Hg

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