Question Video: Constructing a Formula to Determine the Percentage Mass of an Element in a Compound | Nagwa Question Video: Constructing a Formula to Determine the Percentage Mass of an Element in a Compound | Nagwa

# Question Video: Constructing a Formula to Determine the Percentage Mass of an Element in a Compound Chemistry • First Year of Secondary School

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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?

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### 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.

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