Can you solve it? Chemistry
Ten problems that get harder as you go. Each has a hint and a full worked solution, but try properly before you look. Your progress is saved on this device.
- 1Hard
Atoms in a sip
How many atoms are there in 18 g of water (H₂O)? Take Avogadro's number as 6.022 × 10²³ and give your answer as a number of 10²³, so 3.5 × 10²³ would be typed as 3.5.
- 2Hard
Balance the burner
Balance this equation for burning propane: C₃H₈ + __O₂ → __CO₂ + __H₂O. What number goes in front of O₂?
- 3Hard
Oxygen in chalk
What percentage of the mass of calcium carbonate, CaCO₃, is oxygen? Use Ca = 40, C = 12, O = 16.
- 4Hard
Watering down acid
50 mL of 2 mol/dm³ hydrochloric acid is diluted with water to a total volume of 250 mL. What is the new concentration in mol/dm³?
- 5Very hard
The two-proton acid
What volume of 0.5 mol/dm³ sodium hydroxide, in mL, exactly neutralises 25 mL of 0.2 mol/dm³ sulfuric acid, H₂SO₄?
- 6Hard
Electrons in rust
Iron has atomic number 26. How many electrons does an Fe³⁺ ion have?
- 7Hard
Halving and halving
A sample contains 80 g of a radioactive isotope with a half-life of 5 years. How many grams of the isotope are left after 20 years?
- 8Fiendish
The mystery sugar
A compound is 40.0% carbon, 6.7% hydrogen and 53.3% oxygen by mass, and its molar mass is 180 g/mol. How many carbon atoms are in one molecule? Use C = 12, H = 1, O = 16.
- 9Very hard
Pressure cooker
A sealed rigid container holds gas at 2.0 atm and 27 °C. It is heated to 127 °C. What is the new pressure in atm? Give your answer to 2 decimal places.
- 10Fiendish
Running out of oxygen
8 g of methane (CH₄) burns with only 16 g of oxygen: CH₄ + 2O₂ → CO₂ + 2H₂O. What is the greatest mass of carbon dioxide that can form, in grams? Use C = 12, H = 1, O = 16.