Question

Calculate the mass of ethyl alcohol which must be dissolved in 750 grams of water to make 0.5m solution.

Answer 1

The molar concentration $c$ of ethyl alcohol solution is the ratio of the number of the moles of ethyl alcohol $n$ and the volume of the solution $V$.

$c =\frac{n}{V}$ .

The density of 0.5 M solution of ethanol at 20°C is 0.9939 g/mL. The molar mass $M$ of ethanol is 46.07 g/mol. Therefore:

$c = \frac{m_{eth}/M}{(m_w+m_{eth})/d}$

$0.5 = \frac{m_{eth}/46.07}{(750+m_{eth})/0.9939/1000}$

$m_{eth} = 17.8$ g.

Answer: 17.8 g of ethyl alcohol must be dissolved in 750 grams of water to make 0.5M solution at 20°C.