Library / Foundations: Chemistry / FCH-05
Foundations: Chemistry
Reacting masses from an equation
Find the mass of a product from the mass of a reactant and the balanced equation.
- Ref
- FCH-05
- Rev
- v1
- Field
- Foundations: Chemistry
- Result
- mB [g]
For the balanced reaction aA → bB, a and b are the coefficients, the middle ratio is the mole ratio from the equation and M is the relative formula mass of each species.
Change the known mass to moles, use the equation to turn moles of one substance into moles of another, then change back to mass.
Calculator
- Moles of A0.1 mol
- Volume of B if a gas at RTP2.4 dm³
What drives the result
Each variable is moved across its typical range while the others stay at the values above. The longest bar is the key driver.
- The balanced equation fixes the ratio of moles, not masses.
- 2.40 g of magnesium makes 0.1 mol of hydrogen, which is 0.2016 g, or 2.4 dm³ at room temperature and pressure.
Chart data as tables
Ranked variables
| Variable | Typical range | Lowest result | Highest result | Swing (g) | Local step | Change in result |
|---|---|---|---|---|---|---|
| mA Mass of A | 0.01 to 1,000 g | 8.4e-4 | 84 | 84 | +10% | +10 % |
| MB Mr of B | 1 to 500 g/mol | 0.1 | 50 | 49.9 | +10% | +10 % |
| MA Mr of A | 1 to 500 g/mol | 0.009677 | 4.838 | 4.829 | +10% | −9.09 % |
| b Coefficient of B | 1 to 10 | 0.2016 | 2.016 | 1.814 | +10% | +10 % |
| a Coefficient of A | 1 to 10 | 0.02016 | 0.2016 | 0.1814 | +10% | −9.09 % |
Sweep of Mass of A
| Mass of A (g) | Mass of product B (g) |
|---|---|
| 0.01 | 8.4e-4 |
| 0.04217 | 0.003542 |
| 0.1778 | 0.01494 |
| 0.7499 | 0.06299 |
| 3.162 | 0.2656 |
| 13.34 | 1.12 |
| 56.23 | 4.724 |
| 237.1 | 19.92 |
| 1,000 | 84 |
Sweep one variable
Variables and how each one contributes
Mass of product B
What the equation gives you. Everything else feeds it.
Mass of A
Mass of reactant A (Mg). Directly proportional.
Mr of A
Molar mass of A (Mg = 24). Inversely proportional.
Coefficient of A
Balancing number in front of A. Inversely proportional.
Coefficient of B
Balancing number in front of B (H₂ in Mg + 2HCl → MgCl₂ + H₂ is 1). Directly proportional.
Mr of B
Molar mass of B (H₂ = 2.016). Directly proportional.
Worked example with the values above
- Moles of A: 2.4 / 24 = 0.1 mol.
- Moles of B: 0.1 × (1/1) = 0.1 mol.
- mB = 0.1 × 2.016 = 0.2016 g.
Assumptions and limits
- Reaction goes to completion.
- Reactant A is the limiting reagent.
Common mistakes
- Using the unbalanced equation.
- Using masses directly instead of moles in the ratio.
Statistics and uncertainty
Errors in each measurement and in the assumed completeness of the reaction add up, which is why real yields are below the theoretical value.
Where it is used
- Planning reagent quantities.
- Gas generation and propellant calculations.
Maths you need
- Balanced equations: coefficients
- Moles: n = m / M