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Stoichiometry Workbook 2 Mixture Solute : The compound that is dissolved in the mixture Solvent : The substance that the solute is dissolved in. 𝑀𝑜𝑙𝑎𝑟𝑖𝑡𝑦 = 𝑚𝑜𝑙𝑒𝑠 𝑜𝑓 𝑠𝑜𝑙𝑢𝑡𝑒 𝑣𝑜𝑙𝑢𝑚𝑒 𝑜𝑓 𝑠𝑜𝑙𝑢𝑡𝑖𝑜𝑛 𝑖𝑛 𝑑𝑚3 Solve the problems below: 1. What is the molarity of a solution in which 58 g of NaCl are dissolved in 1.0 dm3 of solution? Process: It is required to find out the number of moles of the solute that are dissolved in 1 .0 dm3 of solution. For this reason we have to convert the mass of solute given to us in moles: (𝑀𝑟(𝑁𝑎𝐶𝑙)=58.5 𝑔/𝑚𝑜𝑙 therefore 58.5g of NaCl (s) correlate to 1 mole of NaCl 58 g of NaCl (s) correlate to x mole of NaCl 𝑥= 58 𝑔 𝑥 1 𝑚𝑜𝑙 58.5 𝑔 = 0.99 mol Since we have 0.99 mol dissolved in 1.0 dm3 of solution, the solution has a concentration (or molarity)of 0.99 M. (***The unit is pronounced as Molars – NOT to be confused with moles! 1 molar = 1M = 1 mol/dm3) 2. What is the molarity of a solution in which 10.0 g of AgNO3 is dissolved in 500 cm3 of solution? 3. How many grams of KNO3 should be dissolved in 2.00 dm3 of solution in order to prepare a 0.500M solution? 4. To what volume should 5.0g of KCl be diluted to in order to prepare a 0.25 M solution? 5. How many grams of CuSO4•5H2O are needed to prepare a 100 cm3 of an aqueous CuSO4 solution with concentration of 0.10M? (Advanced question) Composition of hydrates: Hydrates are ionic compounds with water molecules mechanically traped within their crystal latice. The water is in a specific ratio to each formula unit of the salt. For example, the molecular formula for coper sulfate pentahydrate: CuSO4•5H2O, indicates that there are 5 water molecules present for each one formula unit of CuSO4. Answer the following questions: 1. What percentage of water is found in CuSO4•5H2O? Solution: The hydrated salt hasa total molecular mass (including water) of 249.7 g/mol The mass of the water present is 90.0 g/mol (5 x 18 g/mol) The percentage of water present is: I In 249.7 g/mol of salt there are 90 g/mol of water present f 100.0 g/mol of salt are assumed then X g/mol of water present 𝑥= 100.0 𝑥 90.0 249.7 = 36.1 g/mol A 36.1 % of water is present in the molecule. 2. What percentage of water is found in Na2S•9H2O? 3. A 5.0 g of a hydrate of BaCl2 was heated and only 4.3 g of the anhydrous salt remained. What percantage of water was in the hydrate? 4. A 2.5g sample of a hydrate of Ca(NO3)2 was heated and only 1.7 g of the anhydrated salt remained. What percentage of water was in the hydrate? 5. A 3.0 g sample of Na2CO3•H2O is heated to constant mass. How much anhydrous salt remains? 6. A 5.0 g sample of Cu(NO3)2•xH2O is heated to consatnt mass and only 3.9g of the anhydrate salt reamins. What is the value of x? Mole-Mole Problems 1. N2 (g)+ 3H2 (g) →2NH3 (g) How many moles of hydrogen are needed to completely react with two moles of nitrogen? 2. 2KClO3(s) → 2KCl (s) + 3O2 (g) How many moles of oxygen are produced by the decompostition of six moles of potassium chlorate? 3. Zn (s) + HCl (aq) → ZnCl2 (s) + H2 (g) How many moles of hydrogen are produced from the reaction of three moles of zing with an excess of hydrochloric acid? 4. C3H8(g) + 5O2(g) → 3 CO2(g) + 4 H2O(l) How many moles of oxygen are necessary to react completely with 4 moles of propane (C3H8) 5. K3PO4 (aq) + Al(NO3)3(aq) →3KNO3 (aq) + AlPO4(aq) How many moles of potassium nitrate are produced when two moles of potassium phosphate react with two moles of aluminium nitrate?