To calculate mass to mass in chemistry, you need to perform a series of conversions using molar mass and mole ratios derived from a balanced chemical equation.
Here's a step-by-step breakdown:
1. Write and Balance the Chemical Equation:
This is the foundation. Make sure your chemical equation is correctly balanced to represent the stoichiometry of the reaction accurately. This ensures the correct mole ratios are used.
2. Convert the Given Mass of Substance A to Moles:
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Use the molar mass of substance A (grams per mole) to convert from grams of A to moles of A. The molar mass is found by adding up the atomic masses of all the atoms in the substance's formula (found on the periodic table).
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Formula:
Moles of A = Mass of A (in grams) / Molar Mass of A (in g/mol)
3. Determine the Mole Ratio from the Balanced Equation:
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The coefficients in the balanced chemical equation represent the mole ratio between reactants and products.
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For example, in the balanced equation:
2A + B -> 3C, the mole ratio of A to C is 2:3.
4. Convert Moles of Substance A to Moles of Substance B:
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Use the mole ratio to convert from moles of A to moles of B.
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Formula:
Moles of B = Moles of A * (Mole Ratio of B to A)
5. Convert Moles of Substance B to Mass of Substance B:
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Use the molar mass of substance B (grams per mole) to convert from moles of B to grams of B.
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Formula:
Mass of B (in grams) = Moles of B * Molar Mass of B (in g/mol)
Summary Table:
| Step | Action | Formula/Tool |
|---|---|---|
| 1 | Balance the Chemical Equation | Balanced Chemical Equation |
| 2 | Convert Mass of A to Moles of A | Moles of A = Mass of A / Molar Mass of A |
| 3 | Determine Mole Ratio (A to B) | Coefficients from balanced equation |
| 4 | Convert Moles of A to Moles of B | Moles of B = Moles of A * (Mole Ratio of B to A) |
| 5 | Convert Moles of B to Mass of B | Mass of B = Moles of B * Molar Mass of B |
Example:
Consider the following reaction:
N2 (g) + 3H2 (g) -> 2NH3 (g)
If you start with 10.0 grams of N2, how many grams of NH3 can you produce?
- Balanced Equation: Already balanced (above).
- Moles of N2: Molar mass of N2 = 28.02 g/mol. Moles of N2 = 10.0 g / 28.02 g/mol = 0.357 mol
- Mole Ratio: From the balanced equation, the mole ratio of NH3 to N2 is 2:1.
- Moles of NH3: Moles of NH3 = 0.357 mol N2 * (2 mol NH3 / 1 mol N2) = 0.714 mol NH3
- Mass of NH3: Molar mass of NH3 = 17.03 g/mol. Mass of NH3 = 0.714 mol * 17.03 g/mol = 12.16 g
Therefore, 10.0 grams of N2 can produce 12.16 grams of NH3.
By following these steps, you can successfully calculate mass-to-mass conversions in chemistry problems.
