Question

How many grams of NaCl is required to make 200 mL aqueous solution of 5%(w/v) NaCl?

Answer 1

The weight by volume % also denoted as (w/v) % is used as a measurement to find the concentration of solution. This value is calculated from the weight of solute divided by the total volume of solute and solvent.

Complete step by step answer:
The weight by volume % or (w/v) % is very useful to measure the concentration when providing reagents. This term can also be referred to as mass by volume % or (m/v) %. The weight by volume % is calculated from the below:
{\text{% }}\dfrac{{\text{W}}}{{\text{V}}} = \dfrac{{{\text{Mass of solute (g) x 100}}}}{{{\text{Volume (mL)}}}}
It is given that (w/v) % = 5 and volume = 200 mL.
To find mass of NaCl.
Here, sodium chloride with the chemical formula NaCl is the solute.
Now, we need to substitute in the formula to get
${\text{5}} = \dfrac{{{\text{Mass of solute (g) x 100}}}}{{{\text{200}}}}$
Next rearrange the equation, to get the unknown mass of the NaCl.

$${\text{Mass of solute (g)}} = \dfrac{{{\text{200 x 5}}}}{{{\text{100}}}} \\\ \Rightarrow {\text{Mass of solute}} = {\text{ 10 g}} \\\$$

Hence 10 grams of NaCl is required to make 200 mL aqueous solution of 5%(w/v) NaCl.

Note:
The mass of solute must always be expressed in grams (g) and the volume of the solution (solvent + solute) should be in milliliters (mL). Thus, in general (w/v) % is expressed in g/mL (%).
Important unit conversions:
Kilograms (kg) to grams (g)
$\Rightarrow$ 1kg = 1000 g
Litres (L) to millilitres (mL)
$\Rightarrow$ 1L = 1000 mL