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7.7 Mass Calculations for Reactions

7.7 Mass Calculations for Reactions. A mixture of acetylene and oxygen undergoes combustion during the welding of metals. We can calculate the mass of acetylene used to weld a specific mass of metal.

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7.7 Mass Calculations for Reactions

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  1. 7.7 Mass Calculations for Reactions A mixture of acetylene and oxygen undergoes combustion during the welding of metals. We can calculate the mass of acetylene used to weld a specific mass of metal. Learning Goal Given the mass in grams of a substance in a reaction, calculate the mass in grams of another substance in the reaction.

  2. Equations, Mass of A−Mass of B • Given a balanced equation, convert the mass of substance A to substance B by • first, converting the mass of substance A to moles using the molar mass of A; • second, converting moles of substance A to moles of substance B using the mole–mole ratio of B to A in the balanced equation; and • third, converting moles of substance B to grams using the molar mass of B.

  3. Equations, Mass of A−Mass of B Core Chemistry Skill Converting Grams to Grams

  4. Mass N2 − Mass NH3 Suppose we want to determine the mass (grams) of NH3 that can be produced from 32 grams of N2 in the following equation. N2(g) + 3H2(g)  2NH3(g) STEP 1State the given and needed quantities. ANALYZE Given Need THE PROBLEM 32 g N2 mass (grams) NH3 N2(g) + 3H2(g)  2NH3(g)

  5. Mass N2 − Mass NH3 Suppose we want to determine the mass (grams) of NH3 that can be produced from 32 grams of N2 in the following equation. N2(g) + 3H2(g)  2NH3(g) STEP 2 Write a plan to convert the given to the needed quantity (grams). grams moles moles grams N2 N2 NH3 NH3 Molar mass N2 Mole–mole factor NH3/N2 Molar mass NH3

  6. Mass N2 − Mass NH3 Suppose we want to determine the mass (grams) of NH3 that can be produced from 32 grams of N2 in the following equation. N2(g) + 3H2(g)  2NH3(g) STEP 3 Use coefficients to write mole–mole factors; write molar mass factors if needed.

  7. Mass N2 − Mass NH3 Suppose we want to determine the mass (grams) of NH3 that can be produced from 32 grams of N2 in the following equation. N2(g) + 3H2(g)  2NH3(g) STEP 4Set up the problem to give the needed quantity (grams). • Exact • Exact • Four SFs × × × • Two SFs • Four SFs • Exact • Exact • Two SFs

  8. Study Check How many grams of O2 are needed to produce 45.8 grams of Fe2O3 in the following reaction? 4Fe(s) + 3O2(g)  2Fe2O3(s) A. 38.4 g of O2 B. 13.8 g of O2 C. 1.38 g of O2

  9. Solution How many grams of O2 are needed to produce 45.8 grams of Fe2O3 in the following reaction? 4Fe(s) + 3O2(g)  2Fe2O3(s) STEP 1State the given and needed quantities. ANALYZE Given Need THE PROBLEM 45.8 g Fe2O3 mass (grams) O2 4Fe(s) + 3O2(g)  2Fe2O3(s)

  10. Solution How many grams of O2 are needed to produce 45.8 grams of Fe2O3 in the following reaction? 4Fe(s) + 3O2(g)  2Fe2O3(s) STEP 2 Write a plan to convert the given to the needed quantity (grams). grams moles moles grams Fe2O3 Fe2O3 O2 O2 Molar mass Fe2O3 Molar mass O2 Mole–mole factor O2/Fe2O3

  11. Solution How many grams of O2 are needed to produce 45.8 grams of Fe2O3 in the following reaction? 4Fe(s) + 3O2(g)  2Fe2O3(s) STEP 3 Use coefficients to write mole–mole factors; write molar mass factors if needed.

  12. Solution How many grams of O2 are needed to produce 45.8 grams of Fe2O3 in the following reaction? 4Fe(s) + 3O2(g) 2Fe2O3(s) STEP 4Set up the problem to give the needed quantity (grams). The answer is B. • Four SFs • Exact • Exact × × × • Exact • Three SFs • Exact • Three SFs • Five SFs

  13. Study Check The reaction between H2 and O2 produces 13.1 g of water. How many grams of O2 reacted? 2H2(g) + O2(g)  2H2O(g)

  14. Solution The reaction between H2 and O2 produces 13.1 g of water. How many grams of O2 reacted? 2H2(g) + O2(g)  2H2O(g) STEP 1State the given and needed quantities. ANALYZE Given Need THE PROBLEM 13.1 g H2O mass (grams) O2 2H2(g) + O2(g)  2H2O(g)

  15. Solution The reaction between H2 and O2 produces 13.1 g of water. How many grams of O2 reacted? 2H2(g) + O2(g)  2H2O(g) STEP 2 Write a plan to convert the given to the needed quantity (grams). grams moles moles grams H2OH2OO2 O2 Molar mass H2O Mole–mole factor O2/H2O Molar mass O2

  16. Solution The reaction between H2 and O2 produces 13.1 g of water. How many grams of O2 reacted? 2H2(g) + O2(g) 2H2O(g) STEP 3 Use coefficients to write mole–mole factors; write molar mass factors if needed.

  17. Solution The reaction between H2 and O2 produces 13.1 g of water. How many grams of O2 reacted? 2H2(g) + O2(g)  2H2O(g) STEP 4Set up the problem to give the needed quantity (grams). • Exact • Exact • Four SFs × × × • Three SFs • Four SFs • Exact • Exact • Three SFs

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