Moles WS 6



MolarityConcentration of solutions is often measured in molarity, the number of moles of a solute dissolved in one liter of solution.Molarity ( M ) = moles of solute liters of solutionCalculating molarity is easy. Simply divide moles by liters. If the problem gives you grams, first convert to moles by dividing by molar mass. Also, don't forget to convert mL to L.Example : A saline solution contains 0.90 g of NaCl in 100.0 ml. What is the molarity? (Molar mass of NaCl is 58.5 g.)0.90 g (1 mol NaCl) = 0.015 mol NaCl100.0 mL (1 L) = 0.1000 L58.5 g NaCl 103 mLMolarity = 0.015 mol NaCl = 0.15 M NaCl 0.1000 LOR, in one step: 0.90 g (1 mol NaCl) (103 mL) = 0.015 M NaCl 100.0 mL 58.5 g NaCl1 LCalculate the molarity of each of the following solutions:1. 1.0 moles of KCl in 750. mL of solution2. 0.500 moles of MgCl2 in 1.5 L of solution3. 425 g of CuSO4 in 4.00 L of solution4. 0.060 g of NaHCO3 in 1,500. mL of solutionYou can also use the molarity equation to calculate moles (and grams) or volumes (measured in L or mL).If M = mol then M x L = # molesandL = molL MNotice that all three of the equations above are just different algebraic versions of each other. Use them to solve the next few problems, or use standard conversion techniques:Example of # moles = M x L: How many moles are in 250. mL of 2.0 M CaCl2? How many grams is this?Begin with the value you are given in the problem (250. mL), then use molarity of CaCl2 (2.0 M) as a conversion factor.250. mL (1 L) (2.0 mol CaCl2) = 0.50 mol CaCl2 then 0.50 mol CaCl2 (110.08 g CaCl2) = 55.0 g CaCl2103 mL 1L 1 mol CaCl2OR, all in one step:250. mL (1 L) (2.0 mol CaCl2) (110.08 g CaCl2) = 55.0 g CaCl2 103 mL 1L 1 mol CaCl2# 5-8: Calculate the number of grams of the solute in each solution :5. 1.0 L of 0.50 M NaCl6. 500. mL 0f 2.0 M KNO37. 250. mL of 0.10 M CaCl28. 2.0 L of 0.30 M Na2SO4Example of : # L (or mL) = mol MHow many mL contain 6.25 g of 2.00 M CaCl2? Begin with the value you are given in the problem (6.25 g), then use molarity of CaCl2 (2.00 M) as a conversion factor.6.25 g CaCl2 (1 mol CaCl2) = 0.0568 mol CaCl2 then 0.0568 mol CaCl2 (1 L ) (103 mL) = 28.4 mL40062159715500 110.08 g CaCl2 2.00 mol 1 LNote: upside down molarityOR, all in one step:6.25 g CaCl2 (1 mol CaCl2) (1 L ) (103 mL) = 28.4 mL 110.08 g CaCl2 2.00 mol 1 L9. What is the volume (in liters) of a 0.20M solution that contains 0.30 moles of Na2SO4 dissolved in it ?10. How many mL of a 1.25 M solution of HCl contains 0.62 mol of HCl? 11. How many mL of a 2.5 M solution of NaOH contains 18.2 g NaOH?Making Dilutions (Honors only)Chemists often use concentrated (or “stock”) solutions to make dilute solutions. **The number of moles of solute does not change when a solution is diluted.**Number of moles before dilution = Number of moles after dilutionSince moles = Molarity (M) x liters ( V ), then: M1 x V1 = M2 x V2Example : How would you prepare 100. mL of 0.40M MgSO4 from a solution of 2.0 M MgSO4 ?M1=2.0MV1= ????M2 = .40MV2 = 100. mL2.0 M x V1 = 0.40 M x 100. mL V1 = 0.40 M x 100. mL = 20. mL of the concentrated solution. 2.0 MSo, you would measure out 20. mL of the original solution, then add enough water to it to bring the volume to 100. mL. Important: You can do these dilution problems in either mL or L, but the V1 and V2 must both be in the same units (either both mL or both L; don't mix them up).Calculate the volumes you must dilute to make:10. 500. mL of 0.50 M NaCl from 2.0 M original stock solution.11. 2.0 L of 0.20 M MgSO4 from 0.50 M stock solution.12. 50.0 mL of 0.20 M KNO3 from 4.0 M stock solution. ................
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