Relative Formula Mass

Each round has ten questions. Standard starts by asking how many oxygen atoms are in Ca(NO3)2. It then asks for the Mr of H2O, CO2, NaCl and MgO, followed by four formulas with subscripts, three of them using brackets: CaCO3, Mg(OH)2, (NH4)2SO4 and Al2(SO4)3. The last question uses H2SO4. Formula digits appear as subscripts on screen.

Challenge has one atom-count question, two questions chosen from the four simple formulas and the four formulas with subscripts, three of them using brackets. It then asks for the percentage by mass of chlorine in NaCl, another percentage from one of those four formulas and the total Mr on each side of 2H2 + O2 → 2H2O. The order gets harder as you go.

The game shows the Ar values used for each formula. Choose one of four answers, then read the worked sum before you continue. Mr and atom counts have no unit. Percentage answers include a percent sign and use one decimal place. A right answer scores 10 points, with 5 more if you answer within 15 seconds.

In Standard, the clock only removes the speed bonus. The question stays open after the timer reaches zero. In Challenge, an unanswered question locks at zero and counts as wrong. Practice mode turns the clock off. Press 1 to 4 to choose an answer and Enter to continue after the working appears. Your best scores stay on this device, with a separate score for Challenge. A periodic table challenge gives more practice with element symbols and Ar values.

Relative formula mass, written Mr, is the total of the relative atomic masses, or Ar values, for every atom shown in a chemical formula. A formula tells you which elements are present and how many atoms of each one to count. The number after a symbol is a subscript. If a symbol has no subscript, count one atom.

For water, H2O, use two hydrogen atoms and one oxygen atom: Mr = 2 × 1 + 16 = 18. Mr has no unit. It is a relative value, so do not write grams or another mass unit after it. The periodic table pages show the Ar values used by different exam boards.

A bracket groups atoms that repeat together. In Ca(OH)2, the 2 outside the bracket applies to both O and H. The working is 40 + 2 × (16 + 1) = 74. Multiplying only the first element in the bracket gives the wrong total. The same rule applies to the groups in (NH4)2SO4 and Al2(SO4)3.

In a balanced chemical equation, each kind of atom appears the same number of times on both sides. The total Mr of the reactants therefore matches the total Mr of the products when the front numbers are included. For 2H2 + O2 → 2H2O, the reactants total 2 × 2 + 32 = 36. The products total 2 × 18 = 36. This is one way to check conservation of mass.

Percentage by mass compares the mass of one element in a compound with the compound's Mr. Find the Ar of the element, multiply it by the number of those atoms, divide by Mr and multiply by 100. In CaCO3, oxygen contributes 3 × 16 = 48. The percentage is 48 ÷ 100 × 100 = 48.0%. The game rounds this answer to one decimal place.

AQA Chemistry 8462 section 4.3.1.2 and AQA Combined Science 8464 section 5.3.1.2 cover these calculations. Pearson Edexcel 1CH0 and 1SC0 point 1.43 includes Mr and percentage by mass. OCR Gateway Chemistry A J248 point C2.1c includes Mr calculations for species and balanced equations. Read the AQA Chemistry specification, AQA Combined Science specification, Edexcel Chemistry specification, Edexcel Combined Science specification and OCR J248 specification.

This game uses H 1, C 12, N 14, O 16, Na 23, Mg 24, Al 27, S 32, Cl 35.5, K 39, Ca 40, Fe 56, Cu 63.5, Zn 65 and Br 80. These are the rounded AQA and Edexcel GCSE values in our table. OCR tables show some values to one decimal place, such as Mg 24.3 and Ca 40.1. Use the values printed on your own exam paper.

• Read the formula from left to right and list each element once.
• Multiply each Ar by its subscript. An absent subscript means one.
• A number outside brackets multiplies every atom inside them.
• For a percentage, multiply Ar by the count of that element before dividing by Mr.
• Keep the full Mr until the last step, then round the percentage to one decimal place.
• Check that a percentage is below 100% and that Mr has no unit.

Write a short sum before you add. For Mg(OH)2, group the values first: 24 + 2 × (16 + 1). That makes the bracket multiplier easier to see. If mole calculations are your next topic, Mole Ratio Rush uses Mr when it converts a mass into moles.