# Relative Mass

### How can you find the relative mass of an object given the mass in grams? For example quarters, nickels, dimes etc.

Relative molecular mass definition at Dictionary.com, a free online dictionary with pronunciation, synonyms and translation. Relative mass is the mass of an atom or molecule relative to that of 1/12 of a carbon-12 atom. Under this scheme, a neutral hydrogen atom has a mass of 1. You can think of this as counting each proton or neutron as 1 and ignoring the masses of electrons because they are so small in comparison. So the formula for relative atomic mass is simply.

## 1 Answer

You divide the mass of the object by the mass of the comparison object.

#### Explanation:

**Relative values**

- The relative atomic mass of an atom is the average mass of one atom of that element compared to 1/12 of the mass of one carbon-12 atom. Basically, it is not practical for scientists to use actual masses of atoms in scientific calculations since atoms have very small masses.
- The relative atomic mass of water molecule is 18g/mol. Because, The atomic mass of Hydrogen atom is, H=1. The atomic mass of oxygen atom is, O=16. The formula of water molecule is H2O.

Let's take an easy example. What are the relative values of quarters, dimes, nickels in terms of cents?

You divide every value by the value of a cent. Then,

The result tells us that the relative value of a quarter is 25 times that of a cent, a dime has ten times the relative value, and a nickel has five times the relative value.

**Relative mass**

Let's say that the masses of these coins are: quarter = 5.67 g; dime = 2.27 g; nickel = 5.00 g; penny = 2.50 g.

What are the masses of these coins relative to the mass of a dime?

We get

Thus, compared to a dime, the relative masses are: quarter = 2.50, nickel = 2.20, and cent = 1.10.

## Related questions

Related Topics: More Chemistry LessonsIn 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.

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

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**.

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

**Example: **

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

**Solution: **

_{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

**Example: **

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

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**Solution: **

_{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

**Example: **

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

**Solution: **

^{+}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?

**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: **

_{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

**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

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

Example:

Find the percent composition by mass of potassium dichromate (K

_{2}

## How To Find The Relative Formula Mass

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.

- Show Step-by-step Solutions

**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?

- Show Step-by-step Solutions

**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?

**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:**

_{r}(MgSO

_{4}•7H

_{2}

## Relative Mass Of Proton Neutron Electron

O) = 24 + 32 + (4 × 16) + (7 × 18) = 246M

_{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 =

## Relative Mass

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

## Definition Of The Relative Atomic Mass

_{2}•2H

_{2}O

c) KAl(SO

_{4})

## Relative Mass Of Neutron

_{2}•12H

_{2}O

- Show Step-by-step Solutions

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