Related Topics: More Chemistry Lessons

In this lesson, we will learn

The following diagram shows how to calculate the relative molecular mass or relative formula mass. Scroll down the page for more examples and solutions.

**What is relative formula mass and relative molecular mass?**

**How to calculate the relative formula mass?**

_{2}. Each molecule contains 2 hydrogen atoms.

The relative mass formula of hydrogen gas is

M_{r}(H_{2}) = 2 × A_{r}(H) = 2 × 1 = 2

_{2}O. Each molecule contains 2 hydrogen atoms and 1 oxygen atom.

The relative mass formula of water is

M_{r}(H_{2}O) = 2 × A_{r}(H) + A_{r}(O) = 2 × 1 + 16 = 18

^{+}Cl^{-}.

The relative mass formula of sodium chloride is

M_{r}(NaCl) = A_{r}(Na) + A_{r}(Cl) = 23 + 35.5 = 58.5

**How to find the Percent Mass of Elements in a compound?**

How to use M_{r} to calculate the **percent mass of an element** in a compound?

_{r}(NH_{4}NO_{3}) = (2 × 14) + (4 × 1) + (3 × 16) = 28 + 4 + 48 = 80

Mass of nitrogen in the formula = 28

Mass of nitrogen as a fraction of the total =

Mass of nitrogen as percentage of total mass

Mass of O as a fraction of the total =

Mass of O as percentage of total mass

**How to find percent by mass and percent composition?**

Example:

Find the percent composition by mass of potassium dichromate (K_{2}Cr_{2}O_{7})

Step 1: Find the molar mass of the compound.

Step 2: Divide the total mass of each element by the molar mass and multiply by 100 to find the % mass.**How find the percent of an element in a compound? **

Finding the percent by mass means finding the mass of the elements in the compound and adding the masses for the total mass.

Example:

You have a 7364 milligrams sample of SO_{2}. How many grams of sulfur are in the sample?

**How to calculate the Mass Percent of an Element in a Compound?**

Mass Percent Composition of an Element in a Compound

To calculate the mass percent composition (or simply, the mass percent) of an element in a compound, we divide the mass of the element in 1 mol of the compound with the mass of 1 mol of the compound and multiply by 100%.

Examples:

1. What is the mass percent of carbon in carbon dioxide?

2. What is the mass percent of oxygen in carbon dioxide?**How to find the Percent Mass of Water in a compound?**

How to use M_{r} to calculate the **percent mass of water** in a compound?

_{r}(MgSO_{4}•7H_{2}O) = 24 + 32 + (4 × 16) + (7 × 18) = 246

M_{r}(H_{2}O) = 2 × A_{r}(H) + A_{r}(O) = 2 × 1 + 16 = 18

Mass of water in the formula = 18 × 7 = 126

Mass of water as a fraction of the total =

Mass of hydrogen as percentage of total mass

**How to calculate the Water of Crystallization?**

This video outlines how to determine the percent, by mass, of water trapped in a hydrate's crystal lattice.

Example:

What is the percent of water, by mass, in the hydrate CuSO_{4}•2H_{2}O
**How to calculate the percentage of water in the formula of a hydrated compound?**

Example:

Determine the % water in the following hydrates:

a) CuSO_{4}•5H_{2}O

b) CaCl_{2}•2H_{2}O

c) KAl(SO_{4})_{2}•12H_{2}O

In this lesson, we will learn

- how to calculate the relative formula mass or relative molecular mass
- how to use the relative molecular mass to calculate the percent mass of an element in a compound
- how to use the relative molecular mass to calculate the percent mass of water in a compound

The following diagram shows how to calculate the relative molecular mass or relative formula mass. Scroll down the page for more examples and solutions.

The **relative formula mass** of a substance is the sum of the relative atomic masses of the elements present in a formula unit. The symbol for relative formula mass is M_{r}.

If the substance is made of simple molecules, this mass may also be called the **relative molecular mass**.

**Example: **

What is the relative mass formula of hydrogen gas? (Relative atomic mass: H = 1)

**Solution: **

The relative mass formula of hydrogen gas is

M

**Example: **

What is the relative mass formula of water? (Relative atomic masses: H = 1, O = 16)

**Solution: **

The relative mass formula of water is

M

**Example: **

What is the relative mass formula of sodium chloride? (Relative atomic masses: Na = 23, Cl = 35.5)

**Solution: **

The relative mass formula of sodium chloride is

M

How to use M

**Example: **

What percentage of the mass of ammonium nitrate is nitrogen? (The formula for ammonium nitrate is NH_{4}NO_{3}, Relative atomic masses: H = 1, O = 16, N = 14)

**Solution: **

Mass of nitrogen in the formula = 28

Mass of nitrogen as a fraction of the total =

Mass of nitrogen as percentage of total mass

**Example: **

What percentage of the mass of O in NaNO_{3}? (Relative atomic masses: Na = 23, O = 16, N = 14)

**Solution: **

M_{r}(NaNO_{3}) = 23 + 14 + (3 × 16) = 23 + 14 + 48 = 85

Mass of O as a fraction of the total =

Mass of O as percentage of total mass

Example:

Find the percent composition by mass of potassium dichromate (K

Step 1: Find the molar mass of the compound.

Step 2: Divide the total mass of each element by the molar mass and multiply by 100 to find the % mass.

Finding the percent by mass means finding the mass of the elements in the compound and adding the masses for the total mass.

Example:

You have a 7364 milligrams sample of SO

Mass Percent Composition of an Element in a Compound

To calculate the mass percent composition (or simply, the mass percent) of an element in a compound, we divide the mass of the element in 1 mol of the compound with the mass of 1 mol of the compound and multiply by 100%.

Examples:

1. What is the mass percent of carbon in carbon dioxide?

2. What is the mass percent of oxygen in carbon dioxide?

How to use M

**Example: **

What percentage of the mass of magnesium sulfate is water? (Given that the formula for magnesium sulfate is MgSO_{4}•7H_{2}O, Relative atomic masses: H = 1, O = 16, S = 32, Mg = 24)

**Solution:**

M

Mass of water in the formula = 18 × 7 = 126

Mass of water as a fraction of the total =

Mass of hydrogen as percentage of total mass

This video outlines how to determine the percent, by mass, of water trapped in a hydrate's crystal lattice.

Example:

What is the percent of water, by mass, in the hydrate CuSO

Example:

Determine the % water in the following hydrates:

a) CuSO

b) CaCl

c) KAl(SO

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