Question

Calculate the $(w/W)\% \,of\,10g\,$ of potassium hydroxide in $40g$ of solvent.
A.$25\% $
B.$20\% $
C.$30\% $
D.$40\% $

Answer 1

Hint: $w/W\% $ is a way to express the concentration of a particular component in the solution. It is also called the mass percentage of the component in the solution. A solution is made up of two components- solvent and solute. Solvent is the part that is present in major quantity and solute is the part that is present in minor quantity.

Formulae used:
$Mass\% \,of\,a\,component = \dfrac{{Mass\,of\,the\,component\,in\;the\,solution}}{{Total\,mass\,of\,the\,solution}} \times 100$

Complete step by step answer:
We know that the solution is a homogenous mixture of two or more components. The component which is present in excess is called solvent and the other that is present in low amounts is called solute. We can describe the composition of any solution by expressing its concentration. Now this concentration can be expressed qualitatively or quantitatively. If any solution is dilute (having very low quantity of solute) or concentrated (having high quantity of solute), it means that we are expressing concentration qualitatively. But such expressions can be often misleading. So we need to express concentration quantitatively. There are many ways of expressing concentration quantitatively. Mass percentage $(w/W)$ is one of them.
Mass percentage of a component is defined as
$Mass\% \,of\,a\,component = \dfrac{{Mass\,of\,the\,component\,in\;the\,solution}}{{Total\,mass\,of\,the\,solution}} \times 100$
Now in The above question it is given that we have to dissolve potassium hydroxide in solvent. The mass of component potassium hydroxide is $10g$ and the total mass of the solution is $40g$.
Now put this value in the above equation-
$Mass\% \,of\,KOH = {{10}}{{40}} \times 100 = 25\% $

So the answer is option A.

Note: Mass percentage of a component in a solution is usually used to describe concentration in chemical industries. Since mass percentage is a percent of components in the solution, it has no unit. The molecular formula of potassium hydroxide is $KOH$.