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Solving Stoichiometry Problems

Solving Stoichiometry Problems. Did you know ?. Considered individually, the chemical elements in your body, such as carbon, hydrogen, oxygen, etc., are worth about $20. However, when combined into complex molecules, such as fluids, tissues and antibodies, your body is worth over $45 million!.

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Solving Stoichiometry Problems

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  1. Solving StoichiometryProblems

  2. Did you know? • Considered individually, the chemical elements in your body, such as carbon, hydrogen, oxygen, etc., are worth about $20. • However, when combined into complex molecules, such as fluids, tissues and antibodies, your body is worth over $45 million!

  3. In the lab, chemists use balances to measure chemicals in grams. Therefore, it is necessary to establish a method for converting a measured mass in grams to a chemical equation, which uses moles as quantities. • As in mole-mole calculations, the unknown can be either a reactant or a product.

  4. Review • A balanced equation is essential for all calculations involving amounts of reactants and products. • Problems involving moles and masses can be solved in basically the same way as mole-mole problems. • In a mole-mass problem you are asked to calculate the mass of a substance that will react with, or be produced from, a given number of moles of a second substance. Moles A  Moles B  Mass B

  5. Tip • Follow these steps to solve mole-mass problems: • Write the balanced equation • Determine what is given and what you are looking for (unknown) • Convert moles of the given substance into moles of the unknown • Convert moles of the unknown into grams

  6. Example • Propane is a gas that is commonly used in BBQs. Its formula is C3H8. Calculate the mass of water that is produced by the combustion of 13.8 mol of propane. • Write the balanced equation • Determine what is given and what you are looking for (unknown). • Unknown: mass of water produced • Given: amount of propane = 13.8 mol • Convert moles of the given substance into moles of the unknownFrom the equation: • Convert moles of the unknown into grams • So, 2434 g of water can be formed from 13.8 mol of propane.

  7. In a mass-mole problem, you are asked to calculate the moles of a substance that will react with, or be produced from, a given number of grams of a second substance. Moles A  Moles B  Mass B

  8. Tip • Follow these steps to solve mole-mass problems: • Write the balanced equation • Determine what is given and what you are looking for (unknown). • Convert grams into moles • Convert moles of the given substance into moles of the unknown

  9. Example 2 • How many moles of chlorine are required to react completely with 48.6 g of sodium to produce sodium chloride? • Write the balanced equation • Determine what is given and what you are looking for (unknown). • Unknown: moles of chlorine • Given: mass of sodium = 48.6 g • Convert grams into moles • Convert moles of the given substance into moles of the unknown • So, 0.416 mol of propane can react with 48.6 g of sodium.

  10. Check Your Understanding • How many moles of potassium chloride are produced when sufficient chlorine reacts with 78.9 g of potassium iodide? 2KI + Cl2 I2+ 2KCl • 0.26 mol • 0.48 mol • 0.59 mol • 0.98 mol

  11. Most often, you are given a starting mass and asked to find out the mass of a product. This type of problem is called a mass-mass problem, since you start with a mass and end with a mass. Even though both the given and unknown are in grams, you must convert to moles before using the chemical equation. Mass A  Moles A  Moles B  Mass B

  12. Tip • Follow these steps to solve mass-mole problems: • Write the balanced equation • Determine what is given and what you are looking for (unknown). • Convert grams into moles • Convert moles of the given substance into moles of the unknown • Convert moles of the unknown into grams

  13. Example 3 • Calculate how many grams of ammonia can be produced when 206.9 g of nitrogen reacts with hydrogen. • Write the balanced equation • Determine what is given and what you are looking for (unknown). • Unknown: mass of ammonia • Given: mass of nitrogen = 206.9 g • Convert grams into moles • Convert moles of the given substance into moles of the unknown • Convert moles of the unknown into grams • So, 206.9 g of nitrogen can produce 251.71 g of ammonia.

  14. Check Your Understanding • Carbon monoxide is a toxic gas that can be produced by incomplete combustion in automobiles. 2CO + O2 2CO2 • 625 g • 7.36 g • 9.85 g • 16.0 g

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