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Molarity Molarity expresses a concentration of a solution in moles per liter. The symbol for molarity is “M”. To calculate the molarity of a solution you need to know two things (1) the moles of solute and (2) the volume of solvent in liters. When making solutions, our scales do not automatically calculate and weigh out “moles” of something, so you must use your chemistry skills and convert moles to grams. Do this by calculating molar mass. Example A: If you are given a 0.20 M salt solution, what does this mean? This means there are 0.20 moles in 1 liter of solution Also expressed as 0.20 moles NaCl 1.0 L solution Example B. If you wanted to make 3.0 liters of a 0.20M salt solution, how many grams of sodium chloride would you need? Step 1: Figure out how many moles of NaCl you would need 0.20 M = x moles of NaCl x 0.6 moles of NaCl 3.0 L solution Step 2: Convert moles to grams. Calculate molar mass of solute 58.44 g X 0.6 moles NaCl = 35.06 g of NaCl 1 mole NaCl Using dimensional analysis: 0.20 moles 3.0 L 58.44 g = 35.06 g of NaCl 1L 1 mol Example C: How many liters of a 5.7 M NaCl solution will contain 5.2 grams of salt? Step 1: Figure out how many moles of NaCl you have 5.2 g X 1mole = 0.09 moles 58.44g Step 2: Divide moles by molarity to calculate volume of solution. Remember that 5.7 M means 5.7 moles / 1 L . This also means that 1 L / 5.7 moles 1L X 0.09 moles = 0.02 Liters 5.7 moles Using dimensional analysis: 1L 1 mol 5.2 g = 0.02 Liters 5.7 mol 58.44 g Molarity Worksheet 1 1. How many moles of NaOH do you need in order to make 6.0 liters of a 2.0M solution? 2. To make 20.0 liters of a 0.50M solution of KCl, how many grams of KCl will be needed? 3. How many liters of a 8.2M NH4NO3 solution will contain 5.0 x 10-4 grams? 4. If 3.0 liters of Cl2 gas, at (STP) is dissolved in 75.0 liters of H2O, what is the molarity of the solution? 5. Given 30.0 grams of CuSO4, what is the molarity (hint: density of water is 1.00g/mL), if you mixed this with 540 grams of water? Neglect the volume of CuSO4. 6. You are making a tincture of Iodine. How many liters of ethyl alcohol would you need to make a 0.50M solution if you have 3.0g Iodine? 7. In how many grams of water would 2.0 x 105g of NaCl have to be dissolved to make a 0.20M solution? 8. What is the molarity of a solution that contains 0.173g of K2SO4 dissolved in 200.0 mL of water? 9. To make a 0.30M solution of a Na2CO3, how many moles of Na2CO3 would you have in 3.0 x 103 mL? 10. How many grams of KClO3 would you need in order to make 8.00 liters of a 3.50M solution?
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