# Question #76a7e

##### 1 Answer

Here's what I got.

#### Explanation:

Interestingly enough, the value you cited for the percent composition of iron in hemoglobin is off by an order of magnitude. The actual percent composition of iron in hemoglobin is approximately

The problem provides you with the **percent composition** of iron in hemoglobin, so the first thing that you can do here is use this information to find the *mass* of iron present in

#0.372color(red)(%) quad "Fe" => "0.372 g of Fe for every" quad color(red)("100 g") quad "of hemoglobin"#

You can thus say that your sample contains

#2 color(red)(cancel(color(black)("g hemoglobin"))) * "0.372 g Fe"/(100color(red)(cancel(color(black)("g hemoglobin")))) = "0.00744 g Fe"#

Now, in order to find the number of **atoms** of iron present in the sample, you must use the fact that **mole** of iron has a mass of

The **molar mass** of iron, i.e. the mass of exactly **mole** of iron, essentially tells you the mass of **atoms** of iron because **mole** of iron must contain **atoms** of iron **Avogadro's constant** here.

#overbrace("55.845 g"/"1 mol Fe")^(color(blue)("molar mass of Fe")) stackrel(color(white)(acolor(red)("1 mole Fe" = 6.022 * 10^(23) quad "atoms Fe")aaa))(->) "55.845 g"/(6.022 * 10^(23) quad "atoms Fe")#

This means that your sample contains

#0.00744 color(red)(cancel(color(black)("g"))) * (6.022 * 10^(23) quad "atoms Fe")/(55.845color(red)(cancel(color(black)("g")))) = color(darkgreen)(ul(color(black)(8 * 10^(29) quad "atoms Fe")))#

The answer is rounded to one **significant figure**, the number of sig figs you have for the mass of hemoglobin.