Step-by-step explanation:
First, we need to know that percentage by mass is calculated using % mass of the solute (m1) in relation to the mass of the entire solution (m = m1 + m2).
We use the following formula:
percentage by mass = (mass of solute/mass of solution) * 100
In this case, we know:
percentage by mass = 12%
mass of solute = ?? we want to know (solute is NaCl)
mass of solution: 140 g
So:
12 = (mass of solute/140) x 100
mass of solute = (12 x 140)/100
mass of solute = 16.8 g
Answer: 16.8 g