If a compound contains 40% carbon by mass, an imaginary 100 g sample contains 40 g of carbon. This lets you turn percentages into masses without changing the proportions.
Divide the mass of each element by its relative atomic mass (Ar). Divide those relative amounts by the smallest, then find the simplest whole-number ratio. When percentages have been rounded, the results may only be close to whole-number ratios.
A molecular formula tells you the actual numbers of atoms in a molecule. To get its empirical formula, divide all its subscripts by their highest common factor. You can only simplify when there is a common factor greater than one.
A precipitate can form when two solutions react. In a sealed flask, the total mass remains constant provided no matter enters or escapes. The new solid contains atoms that were already in the solutions.
Worked example
A compound is 40.00% carbon, 6.67% hydrogen and 53.33% oxygen by mass. In 100 g, the relative amounts are 40/12, 6.67/1 and 53.33/16. These give approximately 1:2:1, so the empirical formula is CH2O.
Ethane has molecular formula C2H6. Divide both subscripts by 2 to get the empirical formula CH3.
If a sealed flask and its contents weigh 156.40 g before mixing two solutions, the mass remains 156.40 g after a precipitate forms, provided nothing enters or leaves.
Specification references
Pearson Edexcel GCSE Chemistry 1CH0 Issue 4 (March 2024): 1.44, 1.45a and 1.47a. These unbolded statements apply to Foundation and Higher. These questions are for Edexcel.
Copy these details if you'd like to report a problem. Your answers and history aren't included. Nothing is sent automatically.
Write an explanation
A student mixes two solutions in a sealed flask. A solid precipitate appears, so they say the flask must now weigh more. Explain why that reasoning is wrong.
Compare your explanation with the points below. The text you type isn't marked, saved or sent anywhere.
Compare with the example
Check your explanation for
State that no matter enters or leaves the closed flask.
Explain that the precipitate consists of atoms already present in the solutions.
Conclude that the total mass stays constant despite the change in appearance.
Example answer
The sealed flask still contains the same atoms after the reaction. Some dissolved ions have formed a solid precipitate, but no matter has entered or escaped. The total mass of the flask and its contents stays the same.
An answer to improve
A solid has appeared, so the flask has gained mass.
Improve your answer
Account for all the material inside the flask before and after mixing.
Save your progress
Your progress stays in this browser. Turn on saving if you want to keep it when you leave the page.