Question

What is the molar mass of `Fe_2O_3`, `4H_2O`?

Answer 1

`231.78` `"g/mol"`

Explanation:

We're asked to find the molar mass of iron(III) oxide tetrahydrate, `"Fe"_2"O"_3· 4"H"_2"O"`.

To find the molar mass of a substance, we

1. Refer to a periodic table for the atomic masses of the elements (i.e. iron's mass is `55.85` `color(red)("amu"`); this value is also the molar mass of that element (`"iron" = 55.85` `color(red)("g/mol")`)

2. Multiply the molar mass of each element by however many atoms of that element are in the compound.

3. Sum the total, and that will be the molar mass of the entire compound

(1)

The molar masses of each element in this compound are

• `"Fe"`: `55.85` `"g/mol"`

• `"O"`: `16.00` `"g/mol"`

• `"H"`: `1.01` `"g/mol"`

(2)

Now, let's multiply these values by the number of each element in the compound:

• `"Fe"` : `overbrace((2))^"2 iron atoms in compound"(55.85color(white)(l)"g/mol") = color(blue)(111.70` `color(blue)("g/mol"`

• `"O"`: `overbrace((7))^"3 in iron oxide, 4 in water"(16.00color(white)(l)"g/mol") = color(green)(112.00` `color(green)("g/mol"`

• `"H"`: `overbrace((8))^"8 hydrogen atoms in compound"(1.01color(white)(l)"g/mol") = color(purple)(8.08` `color(purple)("g/mol"`

(3)

Lastly, to find the molar mass, we simply sum the total of the individual elements/ values:

`color(blue)(111.70` `color(blue)("g/mol"` `+ color(green)(112.00` `color(green)("g/mol"` `+ color(purple)(8.08` `color(purple)("g/mol"` `= color(red)(231.78` `color(red)("g/mol"`