Question

How do you calculate moles ?

Answer 1

grams * moles/ grams = moles

Answer 2

Mass-Mole Calculations (moles = mass ÷ molar mass)
Key Concepts
1 mole of a pure substance has a mass equal to its molecular mass expressed in grams.
This is known as the molar mass, M, and has the units g mol-1
So 2 moles of a substance would have a mass = 2 x molar mass
3 moles of a substance would have a mass = 3 x molar mass etc

This leads to the formula: mass = moles x molar mass
If we let:
m = mass of substance in grams,
n = moles of pure substance,
M = molar mass of the pure substance in g mol-1
we can write the equation: m = n x M

This equation can be rearranged to give the following:
n = m ÷ M (moles = mass ÷ molar mass)
M = m ÷ n (molar mass = mass ÷ moles)
Examples
1. mass = moles x molar mass
Calculate the mass of 0.25 moles of water

Write the equation:
m (in grams) = n x M

Extract the data from the question:
n = 0.25 mol

Calculate the molar mass of the substance using the periodic table:
M(H2O) = (2 x 1.008) + 16.00 = 18.016 g mol-1

Substitute the values into the equation and solve:
mass = 0.25 x 18.016 = 4.504 g