Learn: Converting between Mass, Moles, and Mr

Welcome!You've already studied relative formula mass and the mole as the 'amount of substance'. Now, let's build on that by learning how to convert between mass, moles, and Mr. This is a key skill for solving chemical calculations!

What is the relationship between Mass, Moles, and Mr?The relationship can be expressed using the equation: Mass = Moles × Mr. This equation connects the mass of a substance, the number of moles, and the relative formula mass (Mr).It is important because it allows chemists to calculate how much of a substance is needed or produced in a chemical reaction.

Understanding the termsMass is the amount of the substance measured in grams (g).Moles represent the 'amount of substance' and are measured in moles (mol).Relative formula mass (Mr) is the sum of the relative atomic masses of all atoms in a molecule or formula unit.

How to use the formulaTo calculate moles, rearrange the formula to Moles = Mass ÷ Mr. To find Mr, use Mr = Mass ÷ Moles.Remember to always use the correct units (grams for mass, moles for amount, and no units for Mr).

Example Calculation: Converting Mass to MolesLet's say you have 20 g of NaCl. To find the number of moles:1. Calculate the Mr of NaCl (Na = 23, Cl = 35.5, so Mr = 58.5).2. Use the formula: Moles = Mass ÷ Mr = 20 ÷ 58.5 = 0.342 mol.

Example Calculation: Converting Moles to MassIf you have 0.5 moles of CO₂, and the Mr of CO₂ is 44, calculate the mass:1. Use the formula: Mass = Moles × Mr = 0.5 × 44 = 22 g.This tells us the mass of 0.5 moles of CO₂ is 22 g.

Review Time!Great work! You've learned how to calculate mass, moles, and Mr using the formula. Let's test your understanding with a few more questions.