This worksheet provides answers and detailed solutions to common molarity problems. Understanding molarity is crucial in chemistry, allowing us to precisely quantify the concentration of solutions. This key will help you check your work and solidify your understanding of the concepts.
What is Molarity?
Molarity (M) is a measure of concentration, defined as the number of moles of solute per liter of solution. The formula is:
Molarity (M) = moles of solute / liters of solution
Solved Problems:
Here are several molarity problems with step-by-step solutions to guide you.
Problem 1: Calculating Molarity
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Problem: What is the molarity of a solution prepared by dissolving 5.85 grams of NaCl (sodium chloride) in enough water to make 250 mL of solution? The molar mass of NaCl is 58.44 g/mol.
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Solution:
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Convert grams to moles: We need to convert the mass of NaCl to moles using its molar mass.
moles of NaCl = (5.85 g NaCl) / (58.44 g/mol) = 0.100 moles NaCl
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Convert mL to L: The volume needs to be in liters for the molarity calculation.
250 mL * (1 L / 1000 mL) = 0.250 L
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Calculate molarity: Now we can use the molarity formula:
Molarity = (0.100 moles NaCl) / (0.250 L) = 0.400 M
- Answer: The molarity of the NaCl solution is 0.400 M.
Problem 2: Calculating Moles from Molarity and Volume
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Problem: How many moles of glucose (C₆H₁₂O₆) are present in 500 mL of a 0.25 M glucose solution?
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Solution:
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Convert mL to L: First, convert the volume to liters:
500 mL * (1 L / 1000 mL) = 0.500 L
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Calculate moles: Rearrange the molarity formula to solve for moles:
moles of solute = Molarity * Liters of solution
moles of glucose = (0.25 M) * (0.500 L) = 0.125 moles
- Answer: There are 0.125 moles of glucose in 500 mL of a 0.25 M solution.
Problem 3: Calculating Volume from Molarity and Moles
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Problem: What volume (in mL) of a 2.0 M solution of HCl is needed to obtain 0.050 moles of HCl?
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Solution:
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Rearrange the formula: Solve the molarity formula for liters:
Liters of solution = moles of solute / Molarity
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Calculate liters:
Liters of HCl solution = (0.050 moles) / (2.0 M) = 0.025 L
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Convert liters to mL:
0.025 L * (1000 mL / 1 L) = 25 mL
- Answer: You need 25 mL of the 2.0 M HCl solution.
Problem 4: Dilution Problems
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Problem: 100 mL of a 6.0 M stock solution of NaOH needs to be diluted to a concentration of 1.5 M. What will the final volume of the diluted solution be? (Use the dilution formula: M1V1 = M2V2)
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Solution:
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Identify known values: M1 = 6.0 M, V1 = 100 mL, M2 = 1.5 M
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Solve for V2:
V2 = (M1V1) / M2 = (6.0 M * 100 mL) / 1.5 M = 400 mL
- Answer: The final volume of the diluted solution will be 400 mL.
These problems illustrate the fundamental calculations involving molarity. Remember to always pay close attention to units and use the correct formula. Practice is key to mastering these concepts!
