What is Mr. in chemistry?

Mr in chemistry represents the relative formula mass of a substance.

Understanding Relative Formula Mass (Mr)

The relative formula mass (Mr) is a dimensionless quantity that indicates how many times heavier one molecule or formula unit of a substance is than 1/12 of the mass of a carbon-12 atom. In simpler terms, it's the sum of the relative atomic masses (Ar) of all the atoms in a chemical formula.

How to Calculate Mr

Here's a step-by-step guide to calculating the Mr of a substance:

Identify the chemical formula: Determine the chemical formula of the compound you are interested in. For example, water is H₂O, and sodium chloride is NaCl.
Find the relative atomic masses (Ar): Obtain the relative atomic masses (Ar) of each element present in the formula from the periodic table. These values are usually found above the element's symbol.
- For example:
  - Hydrogen (H): Ar ≈ 1
  - Oxygen (O): Ar ≈ 16
  - Sodium (Na): Ar ≈ 23
  - Chlorine (Cl): Ar ≈ 35.5
Multiply and add: Multiply the Ar of each element by the number of atoms of that element present in the formula. Then, add up these values to get the Mr.

Examples

Water (H₂O):
- (2 × Ar of H) + (1 × Ar of O)
- (2 × 1) + (1 × 16) = 2 + 16 = 18
- Therefore, Mr of H₂O = 18
Sodium Chloride (NaCl):
- (1 × Ar of Na) + (1 × Ar of Cl)
- (1 × 23) + (1 × 35.5) = 23 + 35.5 = 58.5
- Therefore, Mr of NaCl = 58.5

Significance of Mr

The relative formula mass is crucial for:

Stoichiometry: Calculating the amounts of reactants and products in chemical reactions.
Molar mass calculations: Mr, expressed in grams, numerically equals the molar mass (grams per mole), which is useful for converting between mass and moles.
Percentage composition: Determining the percentage by mass of each element in a compound.