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Stoichiometry and Limiting Reagants

Stoichiometry and Limiting Reagants. What is Stoichiometry?. A balanced chemical eq’n tells us how much reactants are needed to make a certain number of products. To make ammonia, we react nitrogen gas with hydrogen gas: N 2 (g) + 3H 2 (g)  2NH 3 (g)

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Stoichiometry and Limiting Reagants

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  1. Stoichiometry and Limiting Reagants

  2. What is Stoichiometry? A balanced chemical eq’n tells us how much reactants are needed to make a certain number of products. To make ammonia, we react nitrogen gas with hydrogen gas: N2 (g) + 3H2 (g)  2NH3 (g) The number in front of the element/compound is also known as the mole ratio: 1 mole N2 : 3 moles H2 : 2 moles NH3 If you wanted to make 4 moles of NH3, then you’d need 2 moles N2 and 6 moles H2. Stoichiometry is a way of calculating how much reactants and products are involved in chemical reactions.

  3. Example 1 (Finding mass) Propane, C3H8(g) is used in BBQs. Calculate the mass of oxygen that is needed to burn 15 g of propane.

  4. 2. Find n of given substance nC3H8 = 15.0 g 44.11 g/mol = 0.34 mol 3. Find n of required substance According to the balanced eq’n, for 1 mole of C3H8, 5 moles of O2 is required for the reaction to occur. nO2 = 0.34 C3H8 x 5 mol O2 = 1.7 mol 1 mol C3H8

  5. 4. Find mass of required substance mO2 = 1.7 mol x 32.00 g/mol = 54.4 g Therefore, 54.4 g of oxygen is required to burn 15 g of propane.

  6. Example 2 (Finding # of atoms or molecules) Look at the equation below and balance it. How many hydrogen molecules are produced when 60.0 g of Mg reacts with excess HCl? Mg (s) + 2HCl (aq)→ MgCl2(aq) + H2(g)

  7. 2. Find n of given substance nMg = 60.0 g 24.31 g/mol = 2.47 mol 3. Find n of required substance According to the balanced eq’n, to make 1 mol of H2(g), 1 mol of Mg is required. nH2 = 2.47 Mg x 1 mol H2 = 2.47 mol 1 mol Mg

  8. 4. Find number of molecules using Avogadro’s constant NH2 = 2.47 mol x 6.02 x 1023 molecules 1 mol = 1.49 x 1024 molecules Therefore, 1.49 x 1024 molecules of H2(g) are produced when 60.0 g of Mg reacts with HCl.

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