INTRODUCTION TO CONCENTRATION Practice Problems. You must know the differences among the following terms to be successful making solutions.

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1 1 INTRODUCTION TO CONCENTRATION Practice Problems You must know the differences among the following terms to be successful making solutions. Solution: A solution is a homogeneous mixture in which one or more substances (the solute(s) are dissolved in another substance (the solvent)). Concentration: the amount of a solute in a particular volume or mass of the solution. Amount: how much of a component is present Problem 1: Which of the following are concentrations? (2 pts) a. 5 g/l b. 17 grams c. 84 moles/l d ng/ml e. 4 µg f units/l g. 14 mmoles Problem 2: Describe how to prepare 100 ml of 3 grams / ml KCl (The procedure to prepare this solution is very straightforward and requires no real knowledge of chemistry! You may solve this problem using either proportions or unit cancellation.) (3 pts)

2 2 Problem 3: Which of the following solutions is the most concentrated? Please clearly show your calculations and how you arrived at your answer. (3 pts) 30 mg/l 30 µg/µl Problem 4: How much solute is present in 225 ml of a 50 mg/ml solution of NaCl? (Please clearly show your explanations/calculations.) (3 pts)

3 3 Preparing Laboratory Solutions that Contain One Solute PART A: Percent solutions (Please clearly show your explanations/calculations.) 1. A laboratory solution contains water and ethylene glycol. How could you prepare 150 ml of a 40% (v/v) solution of ethylene glycol? (3 pts) 2. A particular laboratory solution contains 5 µl of propanol per 10 ml of solution. Express this as a percent (v/v) propanol solution. (3 pts)

4 4 PART C: Molarity 1. If the concentration of a solute in a solution is 2.00 X 10-3 moles/liter, how many moles are there in 500 ml of this solution? (3 pts) 2. You add 29.2 g of NaCl to a total volume of 1L. Express the final concentration of this solution in terms of molarity. (FW of NaCl = 58.44) (3 pts) 3. You add 40.0 g of NaCl to a total volume of 500 ml. Express the final concentration of this solution in terms of molarity. (FW of NaCl = 58.44) (3 pts)

5 5 4. How much KCl is present in 250 ml of a 100 mm solution? (FW of KCl = 74.55) (3 pts) 5. How much CaCl2 is required to make up 150 ml of 800 µm CaCl2? (FW of CaCl2 = 111.0) (3 pts)

6 6 PART D: Weight/volume solutions 6. How would you prepare prepare 100 ml of 4%W/V CaCl2? (Please ensure that you show you calculations as part of your explanation.) (3 pts) 7. How would you prepare prepare 250 ml of 4%W/V CaCl2? (Please ensure that you show you calculations as part of your explanation.) (3 pts)

7 7 Dilutions and Concentration of Solutions study guide and practice problems CALCULATING THE CONCENTRATION OF A DILUTED SAMPLE: You have learned to calculate the dilution of a stock concentration. However, dilutions are usually referred to by their concentration, in the same way that a stock solution is. Ex: Suppose you have made a 1/10 dilution of stock solution A. Given that stock solution A is at a concentration of 10 mg NaCl/mL, what is the concentration of the 1/10 dilution of A? To calculate the concentration of the dilution of A, you simply multiply the concentration of stock solution A by the dilution: Dilution = 1/10 X 10 mg NaCl / ml = 1 mg NaCl / ml CALCULATING THE CONCENTRATION OF A STOCK SOLUTION: Sometimes, you will know the concentration of the diluted sample, but will want to calculate the concentration of the stock solution used to prepare the dilution: Ex. Suppose you have a 1 mg / ml NaCl solution. This solution was prepared by taking 1 ml of Stock Solution A and adding 9 ml of water. What is the concentration of the original Stock Solution A? To calculate the concentration of Stock Solution A, you first figure out the dilution: 1 ml stock solution A / 10 ml total = 1/10 dilution Then, multiply the concentration of the dilution by the INVERSE of the dilution to calculate the concentration of Stock Solution A: 1 mg / ml NaCl X 10/1 = 10 mg / ml NaCl (for Stock Solution A)

8 8 Practice problems (Please show your calculations.) 1. If you take 2.5 ml of 5 mg/ml glucose and add 7.5 ml of water, what is the concentration of the resulting diluted glucose solution? (3 pts) 2. If you pipet 10 ml of 100 mm Tris buffer into a graduated cylinder, and bring the total volume of the solution to 150 ml with water, what is the final concentration of the diluted Tris solution? (3 pts)

9 9 3. You have a student helper who made a 20% glycerol solution using the following recipe: 30 ml glycerol stock solution + 60 ml water What is the concentration of the glycerol stock solution that the student helper used? (3 pts) 4. You are working in the lab on a test to determine the protein concentration in a given sample (called A ). You take 50 µl of protein sample A and add 950 µl of buffer to the sample. You then run your experiment on the diluted protein, and get a concentration of 0.1 mg/ml. What is the concentration of protein in the original protein sample A? (3 pts)

10 10 SPECIFICATIONS You can use the examples provided in the Standards and Specifications section of Module 2 part 2 to aid you in solving the following problems. Please show all your calculations. 1. If 18.5 ml of sodium hydroxide (NaOH) solution is required to neutralize g of hydrochloric acid (HCl), what is the molarity of the NaOH solution?the molar mass of NaOH = 40 g/mol. Please show your calculations. (5 pts) 2. A 13 ml sample of HCl is titrated with 22.5 ml of a 0.45M Ca(OH)2 solution. What is the concentration of the HCl solution? (5 pts)