chemistry equations worked examples Flashcards
how to work out relative atomic mass
First work out the relative atomic masses of each element by looking up their mass numbers in the periodic table. Ca = 40, C = 12 and O = 16.
Add these together. Don’t forget to multiply any small numbers by the RAM. From the formula, there is a small 3 written after the oxygen, which means we have three atoms of oxygen in the compound.
Relative formula mass = 40 + 12 + (16 x 3) = 100
how to work out how many moles
How many moles are there in 4 grams of sodium hydroxide (NaOH)?
Use the equation moles = mass / Mr
Work out the Mr (formula mass) by adding together the relative atomic masses (the mass numbers). Na = 23, O = 16 and H = 1 so NaOH = 40
Moles = 4 / 40 = 0.1 mol
percentage yield
In a school laboratory experiment, a teacher combines sulfuric acid with magnesium to produce 6.25 g of magnesium sulfate. If the maximum yield is 8.5 g, calculate the percentage yield in this experiment.
Percentage yield = (actual yield / theoretical yield) x 100
Percentage yield = (6.25 / 8.5) x 100
Percentage yield = 73.5 %
how to work out empirical and molecular formula
A perfume contains a solvents and a mixture of fragrances. A sample of the solvent used in one perfume contains 0.6 g of carbon, 0.15 g of hydrogen and 0.4 g of oxygen. Calculate the empirical formula of the compound used in the solvent.
Step 1: Find the moles of each element using the equation moles = mass / Mr.
Carbon = 0.6 / 12 = 0.05 mol
Hydrogen = 0.15 / 1 = 0.15 mol
Oxygen = 0.4 / 16 = 0.025 mol
Step 2: divide each of the moles by the smallest number calculated. Here, the smallest number of moles is that of oxygen, with 0.025.
Carbon = 0.05 / 0.025 = 2
Hydrogen = 0.15 / 0.025 = 6
Oxygen = 0.025 / 0.025 = 1
Step 3: write out the empirical formula using the ratio calculated. Here we can see that for every atom of oxygen there are two atoms of carbon and six atoms of oxygen. We write this as C2H6O.
how to work out moles, volume and mass
Calculate the moles of sodium chloride in a 100 cm3 solution which has a concentration of 0.5 mol dm-3.
Convert cm3 to dm3 by dividing by 1000.
100 / 1000 = 0.1 dm3
Calculate moles using the equation: moles = concentration x volume
Moles = 0.5 x 0.1 = 0.05 mol
how to work out Molar volume of a gas
What volume does 0.6 moles of ammonia gas occupy at room temperature and pressure?
Use the equation: volume = moles x 24,000 cm 3
Volume = 0.6 x 24,000
Volume = 14,400 cm 3 or 14.4 dm 3
how to work out Molar volume of a gas
What volume does 0.6 moles of ammonia gas occupy at room temperature and pressure?
Use the equation: volume = moles x 24,000 cm 3
Volume = 0.6 x 24,000
Volume = 14,400 cm 3 or 14.4 dm 3
