### Video Transcript

A compound Y contains a known mass
of element X. What is the formula for expressing
the mass of X in compound Y as a percentage of the total mass?

Let’s review how to calculate a
generic percentage by determining what percentage of the dots in the box are
pink. To calculate a percentage, we
divide the part in question by the whole then multiply by 100 percent. In this case, the number of pink
dots represents the part and the total number of dots represents the whole. So the percentage of dots which are
pink is 40 percent.

Now that we know how to calculate a
generic percentage, let’s take a look at the question. We are told that a compound Y
contains a known mass of element X. We need to come up with a formula
that expresses the percentage mass of X in compound Y. Since we want to find the
percentage mass of X, mass of X is the part. As X is a component of compound Y,
compound Y is the whole. But we can’t simply write compound
Y in the denominator of this equation.

Percentages are unitless
values. So in a percentage equation, the
part and the whole must have the same unit in order for the units to cancel. This means that if we use the mass
of X in the numerator, we must use the mass of compound Y in the denominator with
the same units. Then, since we are finding a
percentage, we should multiply by 100 percent. So the formula for expressing the
mass of compound X and compound Y as a percentage of the total mass is percentage
mass of X equals mass of X divided by mass of compound Y times 100 percent.