# One of the chlorophylls, chlorophyll a, has four nitrogen atoms in a molecule, and the mass fraction of nitrogen is 6.27%. What is the relative molar mass of chlorophyll a?

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How many magnesium atoms are in the molecule if the mass fraction of magnesium in chlorophyll is 2.72%?

How many magnesium atoms are in the molecule if the mass fraction of magnesium in chlorophyll is 2.72%?

##### 1 Answer

Here's what I got.

#### Explanation:

The idea here is that you need to use the **percent composition** of nitrogen in chlorophyll a to figure out the **molar mass** of the compound.

You know that chlorophyll a is **by mass** nitrogen, which means for every

To make the calculations easier, let's pick a sample of chlorophyll a that has a mass of exactly **molar mass** of nitrogen to convert the mass to *moles*.

#6.27 color(red)(cancel(color(black)("g"))) * "1 mole N"/(14.0067 color(red)(cancel(color(black)("g")))) = "0.44764 moles N"#

Now, you also know that **every molecule** of chlorophyll a contains **atoms** of nitrogen. This is equivalent to saying that **every mole** of chlorophyll a contains **moles** of nitrogen.

This means that the total number of moles of chlorophyll a present in the sample is equal to

#0.44764 color(red)(cancel(color(black)("moles N"))) * "1 mol chlorophyll a"/(4color(red)(cancel(color(black)("moles N")))) = "0.11191 moles chlorophyll a"#

To find the **molar mass** of chlorophyll a, calculate the mass of exactly **mole** of chlorophyll a. Since you know that **moles** have a mass of **mole** will have a mass of

#1color(red)(cancel(color(black)("mole chlorophyll a"))) * "100. g"/(0.11191color(red)(cancel(color(black)("mole chlorophyll a")))) = "893.58 g"#

Therefore, the **molar mass** of chlorophyll a is

#"molar mass chlorophyll a" = color(darkgreen)(ul(color(black)("894 g mol"^(-1))))#

The answer is rounded to three **sig figs**, the number of sig figs you have for the percent composition of nitrogen.

For the second question, you need to use the fact that every

You've already calculated that **moles** of chlorophyll a, so now use the **molar mass** of magnesium to convert the mass of magnesium present in *moles*.

#2.72 color(red)(cancel(color(black)("g"))) * "1 mole Mg"/(24.305color(red)(cancel(color(black)("g")))) = "0.11191 moles Mg"#

Notice that for every **moles** of chlorophyll a, you get **moles** of magnesium. Since **each mole** of chlorophyll a contains **mole** of magnesium, you can say that **each molecule** of chlorophyll a contains **atom** of magnesium.