To calculate mass in chemical reactions, you'll typically use stoichiometry, a method that relies on the balanced chemical equation and the concept of relative formula mass (Mr).
Steps to Calculating Mass in Chemical Reactions:
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Write the Balanced Chemical Equation: Make sure the equation is correctly balanced to represent the quantitative relationships between reactants and products. This provides the mole ratio needed for the calculation.
- Example: 2H₂ + O₂ → 2H₂O
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Identify the Known and Unknown Substances: Determine which substance's mass is given (the "known") and which substance's mass you need to calculate (the "unknown").
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Calculate the Relative Formula Mass (Mr): Determine the Mr for both the known and unknown substances. This is the sum of the relative atomic masses (Ar) of all the atoms in the formula. You can find Ar values on the periodic table.
- Example: For H₂O, Mr = (2 x Ar of H) + (1 x Ar of O) = (2 x 1) + (1 x 16) = 18
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Write the Known Mass Under its Formula: Indicate the mass of the known substance beneath its corresponding chemical formula in the balanced equation.
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Write the Mr Under its Formula: Indicate the Mr of both the known and unknown substances beneath their chemical formulas in the balanced equation. Adjust these values according to the stoichiometric coefficients in the balanced equation.
- If there are two moles of a substance in the balanced equation, multiply its Mr by 2.
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Apply the Formula: Use the following formula to calculate the unknown mass:
Unknown mass = (Known mass ÷ Total Mr of known substance) × Total Mr of unknown substance
Example Calculation:
Let's say you react 4g of hydrogen (H₂) with excess oxygen (O₂) to produce water (H₂O). How much water is produced?
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Balanced Equation: 2H₂ + O₂ → 2H₂O
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Known: Mass of H₂ = 4g. Unknown: Mass of H₂O.
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Mr:
- Mr of H₂ = (2 x Ar of H) = 2 x 1 = 2
- Mr of H₂O = (2 x Ar of H) + (1 x Ar of O) = (2 x 1) + (1 x 16) = 18
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Write mass under the formula: 2H₂ + O₂ → 2H₂O
4g -
Write Mr under the formula: 2H₂ + O₂ → 2H₂O
4g
2x2 2x18 -
Calculation:
- Total Mr of known substance (H₂) = 2 x 2 = 4
- Total Mr of unknown substance (H₂O) = 2 x 18 = 36
- Mass of H₂O = (4g ÷ 4) × 36 = 36g
Therefore, 36g of water are produced.
Key Considerations:
- Units: Ensure that all masses are in the same unit (e.g., grams).
- Limiting Reactant: If the amounts of multiple reactants are given, identify the limiting reactant. The limiting reactant is the one that is completely consumed and determines the amount of product formed.
- State Symbols: Include the correct state symbols (s, l, g, aq) in your balanced equation.
- Significant Figures: Report your final answer with the appropriate number of significant figures.
Calculating mass in chemical reactions involves using balanced equations and relative formula masses to relate the amounts of reactants and products. By following these steps, you can successfully solve stoichiometry problems.
