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Equations, moles and empirical formulae
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Balanced equations and moles let chemists predict exactly how much product a reaction will make.
Balancing equations
The same number of each atom must appear on both sides. Change the big numbers in front, never the small ones in a formula.
2H2 + O2 → 2H2O
State symbols: (s) solid, (l) liquid, (g) gas, (aq) dissolved in water.
2H2 + O2 → 2H2O
State symbols: (s) solid, (l) liquid, (g) gas, (aq) dissolved in water.
Moles (Higher)
One mole of any substance has mass equal to its M_r in grams.
moles = massM_r
One mole contains 6.02 × 1023 particles (the Avogadro constant).
moles = massM_r
One mole contains 6.02 × 1023 particles (the Avogadro constant).
Empirical formula
The simplest whole-number ratio of atoms.
Divide each mass by its A_r, then divide by the smallest answer.
2.4 g Mg and 1.6 g O: 2.424 = 0.1, 1.616 = 0.1. Ratio 1 : 1, so MgO.
Divide each mass by its A_r, then divide by the smallest answer.
2.4 g Mg and 1.6 g O: 2.424 = 0.1, 1.616 = 0.1. Ratio 1 : 1, so MgO.
How many moles are in 22 g of carbon dioxide (M_r = 44)?
- moles = mass ÷ M_r
- 22 ÷ 44
Answer: 0.5 mol
Balance by changing the numbers in front of formulae. Moles = mass ÷ M_r. Empirical formula: divide masses by A_r, then by the smallest, to get the ratio.
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