Question

An oven-cleaning solution is 40.0% (by mass) NaOH. If one jar of this product contains 454 g of solution, how much NaOH does it contain?

Answer 1

Hint: In this question, the amount of NaOH given is 40%, which means that it constitutes 40% of the total mass of the solution. We are given the total mass of the solution in this question. We just have to convert the percentage to decimal and multiply it to the total mass.

Complete answer:
In this question we are asked to find the mass of NaOH present in 454g of the solution. Here the solution contains NaOH and water, to make the oven cleaning solution.
The percentage by mass of the NaOH dissolved in the solution is $40\% $
Converting the percentage to decimal we get, $40\% = \dfrac{{40}}{{100}} = 0.40$
(This step is very simple to be done by mental calculations)
To find the mass of NaOH in the solution, we need to multiply this by the total mass of the solution i.e. 454 g.
Mass of solute $ = \dfrac{{\% mass}}{{100}} \times total{\text{ }}Mass$
Substituting the values in the above formula we get, Mass of NaOH $ = 0.400 \times 454g$
Mass of NaOH $ = 181.6g{\text{ }}NaOH$
On rounding off to the nearest integer we get the answer as $ \approx 182g$
This is the required answer.

Note:
If we asked to find the no. of moles of NaOH present in 40% of NaOH in 454g of solution, then we’ll convert the above mass into moles.
The molar mass of NaOH is 40 g/mol. The no. of moles can be given by the equation: $Moles = \dfrac{{Mass(g)}}{{Molar{\text{ }}Mass(g/mol)}}$
The no. of moles of NaOH in 182 g is $ = \dfrac{{182}}{{40}} = 4.55mol$
But since we are asked the mass of NaOH which is expressed in grams we’ll end our answer after finding the mass itself.