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A lab manual for CHEM120 Week3 Laboratory 4: Solution Chemistry. The lab aims to teach students how to prepare solutions of specific concentrations and learn about molarity, % concentration, and osmolarity. The lab manual provides instructions on how to calculate solution concentration and prepare solutions. The lab also explains the importance of understanding solution concentration in natural and synthetic processes. The lab manual includes three explorations: molarity, concentration in molarity, and osmolarity. instructions, calculations, and questions for each exploration.

Typology: Exams

2022/2023

1 / 10

Download CHEM120 Week3 Laboratory 4: Solution Chemistry and more Exams Chemistry in PDF only on Docsity! 1 CHEM120 Week3 Laboratory 4: Solution Chemistry Learning Objectives: • To learn how to prepare solutions of specific concentrations • To learn about molarity, % concentration, and osmolarity. In this lab, we will learn how to calculate solution concentration and prepare solutions. For a given solution, different ways of expressing concentration are used depending on our purposes. Solutions made of the same substances but at different concentrations can have very different properties. For example, an animal cell placed in a 0.1% NaCl solution will swell, the same cell placed in a 10% NaCl solution will shrivel or crenate. A good understanding of solution concentration is vital to the understanding of many natural and synthetic processes. Note to Students: • Read the lab before coming to class. The expectation at Chamberlain (CCN/CU) is that you come to class fully prepared for lab. • Always dress properly for lab. Be sure to wear closed toed shoes and long pants to lab. In lab, wear your safety equipment including goggles, lab coat, and gloves. • Always check with your instructor regarding proper waste disposal. • Listen carefully to the professor’s instructions and work safely. • Always follow the rules outlined in the safety contract. • If in doubt of how to use a piece of lab equipment, ask you instructor. Improper use of lab equipment can be dangerous or could damage lab materials. • If you have any safety concerns, see your instructor. • Use deionized water (DI water) in your experiments. Exploration 1: molarity A mole is a specific quantity of units. Much like there are 12 units in a “dozen”, there are 6.022 x 1023 units in a mole. Moles allow us to relate real world quantities and concentrations back to balanced chemical relationships as we studied earlier this week. Molarity is defined as having units of moles/liter. Mathematically, this is expressed as mol/L for molarity and is written in short hand as M. The molarity of a solution can be calculated by dividing the moles of compound in a solution by the total liters of solvent in the solution. Let us look at a situation where you want to calculate the molarity of a solution prepared by dissolving 4.2 grams of LiCl in 250 ml of water. First, convert grams of LiCl to moles of LiCl using the molecular mass of LiCl from the periodic table: 4.2 g LiCl × (1 mole LiCl/ 42.39g LiCl) = 0.1 moles LiCl Since molarity is defined as moles/liter, I also need to convert my 250 ml of solvent to liters: 250 ml × (1 liter/ 1000 milliliters) = 0.25 liter 2 Finally, find the molarity of the solution by dividing moles by liters: 1.1 mol LiCl/0.25 L = 0.4 mol LiCl /L. We can also express this as 0.4 M LiCl. In this first exploration, we will be exploring molarity as we prepare solutions. As a reminder, molarity has units of mol/L. Materials: NaCl, CaCl2, DI water, 100ml beaker, 250 ml beaker, stirring rod NaCl solution- method: 1. Transfer 100 ml of water into your 250 ml beaker. 2. Calculate how many grams of NaCl you require to make 100 ml of a 1 M NaCl solution and write the answer here: 5.84 g NaCl . Show your work in the space below. 3. After checking your calculation in part 2 with your instructor, measure out the correct amount of salt and add it to your 250 ml beaker. 4. Stir until dissolved. 5. Do not dispose of this material at the conclusion of this exploration, as we will use this solution in another part of this lab. CaCl2 solution- method: 1. Transfer 100 ml of water into your 250 ml beaker. 2. Calculate how many grams of CaCl2 you require to make 100 ml of a 1 M CaCl2 solution and write the answer here: 11.1 g CaCl2 . Show your work in the space below. 3. After checking your calculation in part 2 with your instructor, measure out the correct amount of salt and add it to your 250 ml beaker. 4. Stir until dissolved. 5. Do not dispose of this material at the conclusion of this exploration, as we will use this solution in another part of this lab. 5 Concentration in Molarity Mass/Volume % Concentration Original NaCl Solution 1 M NaCl 58.43% Flask A NaCl Solution .2 M NaCl 58.45% Flask B NaCl Solution .05 M NaCl 14.6% Calculations: 6 Questions: 1. Which solution has the greatest number of NaCl molecules/ml of solution? The Solution in Flask A 2. If you wanted to prepare a 50ml solution of 0.05 mM NaCl solution from NaCl and water, what mass of NaCl would you need to measure out? (note: 1000 mM = 1 M). .146 g NaCl 3. How many ml of a 1 M NaCl solution would you need to prepare a 0.05mM NaCl solution? Show all work below ? (note: 1000 mM = 1 M) V1= 2500 mL 4. Consider the two methods of preparing a 0.05 M NaCl solution from the questions above. Which would allow for a more accuare and simple preparation of the 0.05 M NaCl solution? Why? I would say that the first equation would allow for a more accurate and simple preparation for the 0.05 M NaCl solution, because it leaves less room for error. With the second equation, you have to go an extra step further, decide what the concentrations and volumes are, and then convert them into mM. Exploration 3: Osmolarity The concept of osmolarity flows naturally from the concept of molarity. Just as the unit “mole” is used for molarity, the “osmole” is used for osmolarity (Osmol/L). The big difference here is that when we use osmoles, we take into account that a compound can dissociate when dissolved. When 1 mole of KI dissolves in a liter of water, it breaks up into K+ and I- and we count each of these ions separately. If we were to state the molarity of this solution, we would say that it is a 1 M KI solution. However, as the KI dissociates in solution, we would refer to the solution as a 2 Osmol/L solution. As we can see in this example, when a compound does not dissociate when dissolved, osmolarity and molarity of that 7 compound will be the same; however, if the compound dissociates, they will be different. We can put this relationship in a formula as seen below: Osmolarity = molarity × i, where i = number of dissociated ions. In this exploration, we will look at your 1 M solutions of NaCl and CaCl2 in order better understand the Osmole. Materials: 1M NaCl solution from Exploration 1, conductivity meter, DI water bottle, and a waste beaker. Method: 1. Dilute the 1 M solutions to 0.05 M solutions by diluting 1 ml of each solution separately to a total volume of 20 ml. 2. Wash the end of the conductivity meter with DI water over the waste beaker 3. Dip the probe of the conductivity meter into your 0.05 M NaCl solution and record the value in the table below. 4. Wash the end of the conductivity meter with DI water over the waste beaker 5. Dip the probe of the conductivity meter into your 0.05 M CaCl2 l solution and record the value in the table below. 6. Wash the end of the conductivity meter with DI water over the waste beaker and allow to dry on a paper towel. Complete the table below starting with the solution molarity values from exploration 1. Molarity (mol/L) Osmolarity NaCl solution 1 M 2 Osmol/L CaCl2 Solution 1M 3 Osmol/L Questions: 1. Which of the 0.05 M solutions have a greater Osmolarity and conductivity values? What do you think causes these higher values? The .05 NaCl colution has a greater Osmolarity level because of the amount of water available. 2. If a person’s blood had a high osmolarity value, what might this indicate? It means you are dehydrated.