Question

28 How many milliliters of a 4.0% (w/v) solution must be used to prepare 250 mL of a 0.080% (w/v) solution? A 5.0 mL B 2.5 mL 0.50 mL D 25 mL

Answer 1

Let the molecular weight of the solute is M.

100 ml of solution contains 4.0 gm of solute.

Therefore 1000 ml of the solution will contain $\frac{4 \times 1000}{100}=40 gm$ of solute.

Therefore the molarity of the solution is given by $\frac{40}{M}$ (M)

100 ml of the solution contains 0.080 gram of solute.

Therefore 1000 ml of the solution will contain $\frac{0.080 \times 1000}{100}=0.8 gm$ of solute.

Therefore the molarity of the solution is given by $\frac{0.8}{M}$ (M)

Let x ml of 4.0%(w/v) is required to make 250 ml of 0.080%(w/v) solution.From

$\begin{aligned} M_1\times V_1&=M_2\times V_2\\ \frac{40}{M}\times x&=\frac{0.8}{M}\times 250\\ x&=\frac{0.8\times 250}{40}\\ x&=5 \end{aligned}$

5 ml 4.0%(w/v) must be used to prepare 250 ml of 0.080%(w/v) solution.