Chemical Calculations

What Chemical Calculations covers

Chemical Calculations is examined on AS Unit 1 of WJEC Chemistry, The Language of Chemistry, Structure of Matter and Simple Reactions. 161 recall questions across 12 subtopics.

The 12 subtopics

Each one has its own page. WJEC AS Chemistry students sit this topic in the first year.

• Relative masses and the mass spectrometer: relative atomicRecall questions on relative atomic mass.Open →
• Relative masses and the mass spectrometer: how a mass spectrometer worksFifteen recall questions on relative masses and the mass spectrometer: how a mass spectrometer works.Open →
• Relative masses and the mass spectrometer: molecular ionTwelve recall questions on relative masses and the mass spectrometer: molecular ion.Open →
• The mole, the Avogadro constant and molar mass: relative mass terms and simple mass spectraRecall questions on relative mass terms and simple mass spectra.Open →
• The mole, the Avogadro constant and molar mass: moles and percentage composition by massWhy a mole of any element holds the same number of atoms, calculating amounts in moles and masses from them, and working out the percentage by mass of an element in a compound and the mass extractable from an ore.Open →
• Empirical and molecular formulaeRecall questions on Empirical formulae and molecular formulae.Open →
• Concentration, solubility and titration calculations: concentration in mol dm-3 and in g dm-3Recall questions on concentration in mol dm-3 and in g dm-3.Open →
• Concentration, solubility and titration calculations: stoichiometry and the limiting reagentRecall questions on stoichiometry and the limiting reagent.Open →
• Concentration, solubility and titration calculations: titration calculations and estimating a titreRecall questions on titration calculations and estimating a titre.Open →
• Gas volumes and the ideal gas equation24 recall questions on gas volumes and the ideal gas equation.Open →
• Yield, atom economy and experimental uncertainty: concepts of atom economy and percentage yieldRecall questions on concepts of atom economy and percentage yield.Open →
• Yield, atom economy and experimental uncertainty: how to estimate the percentage errorFourteen recall questions on yield, atom economy and experimental uncertainty: how to estimate the percentage error.Open →

What you need to know

Answer each one before you reveal it.

Take Chemical Calculations further

The same content, four other ways.

Questions from this topic

Each one with the model answer underneath.

1. 1

   Define relative atomic mass.

   Model answer

   It is the weighted mean mass of an atom of an element, taking account of the abundances of all its isotopes, compared with one twelfth of the mass of an atom of carbon-12.

2. 2

   Why do relative atomic masses have no units?

   Model answer

   Each one is a ratio of two masses, the mass of an atom compared with one twelfth of the mass of a carbon-12 atom. Dividing a mass by a mass leaves a pure number.

3. 3

   What is relative isotopic mass, and how does it differ from relative atomic mass?

   Model answer

   Relative isotopic mass is the mass of an atom of one particular isotope compared with one twelfth of the mass of a carbon-12 atom, so it refers to a single kind of atom. Relative atomic mass is the weighted average over all the isotopes present, which is why it is rarely a whole number.

4. 4

   Distinguish between relative molecular mass and relative formula mass.

   Model answer

   Relative molecular mass is the sum of the relative atomic masses of all the atoms in one molecule, so it applies only to substances made of discrete molecules. Relative formula mass is the same sum taken over the formula given, and is the term used for giant structures such as ionic solids where no molecule exists.

5. 5

   Why is the relative atomic mass of chlorine 35.5 when no chlorine atom has that mass?

   Model answer

   Chlorine exists as two isotopes, chlorine-35 and chlorine-37, present in a ratio of about 3 : 1. The value 35.5 is the weighted average over the mixture, not the mass of any individual atom.

6. 6

   Calculate the relative formula mass of calcium carbonate, using Ca = 40.1, C = 12.0 and O = 16.0.

   Model answer

   M = 40.1 + 12.0 + (3 × 16.0) = 40.1 + 12.0 + 48.0 = 100.1. Calcium carbonate is ionic, so relative formula mass is the correct term.

What is inside Levo

Six ways to work the same material.

Chemical Calculations revision questions

Is Chemical Calculations examined at AS in WJEC Chemistry?

Yes. It sits in AS Unit 1, which you take in the first year, so it is on the AS paper as well as counting towards the full A level.

What does Chemical Calculations cover in WJEC AS-Level Chemistry?

12 subtopics: relative masses and the mass spectrometer: relative atomic, relative masses and the mass spectrometer: how a mass spectrometer works, relative masses and the mass spectrometer: molecular ion, the mole, the avogadro constant and molar mass: relative mass terms and simple mass spectra, the mole, the avogadro constant and molar mass: moles and percentage composition by mass, empirical and molecular formulae, concentration, solubility and titration calculations: concentration in mol dm-3 and in g dm-3, concentration, solubility and titration calculations: stoichiometry and the limiting reagent, concentration, solubility and titration calculations: titration calculations and estimating a titre, gas volumes and the ideal gas equation, yield, atom economy and experimental uncertainty: concepts of atom economy and percentage yield and yield, atom economy and experimental uncertainty: how to estimate the percentage error. Each one has its own page with questions and model answers.

How is Chemical Calculations organised?

By the WJEC 1410QS specification, subtopic by subtopic, so it matches the order you are taught it in.