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?
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
To make the calculations easier, let's pick a sample of chlorophyll a that has a mass of exactly
#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
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
#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
#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