Question

A certain liquid X has a normal boiling point of 118.40 C and a boiling point elevation constant K,-240 С kg-mol-1. A solution is prepared by dissolving some benzamide (C,H,NO) in 150. g of X. This solution boils at 120.6 C. Calculate the mass of C,H,NO that was dissolved Be sure your answer is rounded to the correct number of significiant digits.

Answer 1

Answer : The mass of benzamide is, 0.166 grams

Explanation :

Formula used for Elevation in boiling point :

$\Delta T_b=i\times k_b\times m$

or,

$T_b-T^o_b=i\times k_b\times \frac{w_2\times 1000}{M_2\times w_1}$

where,

$T_b$ = boiling point of solution = $120.6^oC$

$T^o_b$ = boiling point of liquid X = $118.40^oC$

$k_b$ = boiling point constant  of liquid X = $240^oC/m$

m = molality

i = Van't Hoff factor = 1 (for non-electrolyte)

$w_2$ = mass of solute (benzamide ) = ?

$w_1$ = mass of solvent (liquid X) = 150 g

$M_2$ = molar mass of solute (benzamide ) = 121.14 g/mol

Now put all the given values in the above formula, we get:

$(120.6-118.40)^oC=1\times (240^oC/m)\times \frac{(w_2)\times 1000}{121.14g/mol\times (150g)}$

$w_2=0.166g$

Therefore, the mass of benzamide is, 0.166 grams