molarity problems worksheet answer key

Lemon

2 min read

·

04-02-2025

1

0

Share

Molarity Problems Worksheet: Answer Key & Solution Explanations

This answer key provides solutions and detailed explanations for common molarity problems. Understanding molarity is crucial in chemistry, as it describes the concentration of a solution. Remember, molarity (M) is defined as moles of solute per liter of solution: M = moles/liter.

Problem 1: Calculate the molarity of a solution prepared by dissolving 5.85 g of NaCl in enough water to make 250 mL of solution.

Answer: 0.4 M NaCl

Solution:

1. Find moles of NaCl: First, find the molar mass of NaCl (Na = 22.99 g/mol, Cl = 35.45 g/mol). Molar mass of NaCl = 22.99 + 35.45 = 58.44 g/mol. Moles of NaCl = (5.85 g) / (58.44 g/mol) = 0.1 mol

2. Convert volume to liters: 250 mL * (1 L / 1000 mL) = 0.25 L

3. Calculate molarity: Molarity = moles/liter = 0.1 mol / 0.25 L = 0.4 M

Problem 2: What mass of potassium hydroxide (KOH) is needed to prepare 500 mL of a 0.250 M KOH solution?

Answer: 7.01 g KOH

Solution:

1. Find moles of KOH: Moles = Molarity × Volume (in liters) = 0.250 mol/L * 0.500 L = 0.125 mol KOH

2. Find molar mass of KOH: (K = 39.10 g/mol, O = 16.00 g/mol, H = 1.01 g/mol) Molar mass of KOH = 39.10 + 16.00 + 1.01 = 56.11 g/mol

3. Find mass of KOH: Mass = moles × molar mass = 0.125 mol * 56.11 g/mol = 7.01 g

Problem 3: If you have 2.50 L of a 1.50 M solution of sulfuric acid (H₂SO₄), what volume of this solution would contain 0.500 moles of H₂SO₄?

Answer: 0.333 L (or 333 mL)

Solution:

This problem uses the formula: Moles = Molarity x Volume. We need to rearrange to solve for Volume: Volume = Moles / Molarity

Volume = 0.500 moles / 1.50 mol/L = 0.333 L

Problem 4: How many moles of solute are present in 250 mL of a 0.100 M solution of glucose (C₆H₁₂O₆)?

Answer: 0.025 moles of glucose

Solution:

Moles = Molarity x Volume (in liters) = 0.100 mol/L * 0.250 L = 0.025 mol

Problem 5 (Advanced): A 25.00 mL sample of a solution of HBr is titrated with 0.200 M NaOH. If 35.00 mL of NaOH is required to reach the endpoint, what is the concentration of the HBr solution?

Answer: 0.280 M HBr

Solution:

This is a stoichiometry problem involving a neutralization reaction: HBr(aq) + NaOH(aq) → NaBr(aq) + H₂O(l). The mole ratio of HBr to NaOH is 1:1.

1. Find moles of NaOH: Moles = Molarity x Volume (in liters) = 0.200 mol/L * 0.03500 L = 0.00700 mol NaOH

2. Find moles of HBr: Since the mole ratio is 1:1, moles of HBr = 0.00700 mol

3. Calculate molarity of HBr: Molarity = moles / volume (in liters) = 0.00700 mol / 0.02500 L = 0.280 M

This worksheet covers several types of molarity problems. Remember to always pay close attention to units and use the correct formula (M = moles/liter) and stoichiometric relationships where necessary. Practice is key to mastering molarity calculations!