Moles from mass
To find the number of moles, divide the mass in grams by the relative formula mass. Moles = mass / Mr.
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Analogy
If one box weighs 18 g and you have 90 g of boxes, you have 5 boxes.
Example
Mass = 36 g, Mr = 18Moles = 36 / 18= 2 mol
In practice
It lets you convert what you weigh on a balance into an amount you can use in an equation.
Practice
Flashcards
- What is the formula linking moles, mass and Mr?Moles = mass / Mr.
Interview questions
- Calculate the mass of 0.5 mol of calcium carbonate, CaCO3. (Ar: Ca = 40, C = 12, O = 16)
- Mr = 40 + 12 + 3 x 16 = 100
- Mass = moles x Mr
- 0.5 x 100 = 50 g
Questions people ask
How do you rearrange the moles equation?
Moles = mass / Mr. Rearranged, mass = moles x Mr, and Mr = mass / moles. A triangle with mass on top and moles and Mr along the bottom helps. Cover the one you want and you see what to do with the other two.
How do you use moles to work out the mass of product?
Convert the known mass to moles. Use the ratio from the balanced equation to find the moles of the product. Then multiply by the product's Mr to get its mass. For 2Mg + O2 -> 2MgO, 2 mol of magnesium gives 2 mol of magnesium oxide.