Lab 2: Solutions and Dilutions - St. Lawrence College

CHEM 1001 Lab 2: Solutions and Dilutions PART I: MEASURING MASS Before beginning this lab, there are some terms that must be defined from a scientific perspective: In science we must clearly define the difference between SCALES and BAL...

CHEM 1001 Lab 2: Solutions and Dilutions PART I: MEASURING MASS Before beginning this lab, there are some terms that must be defined from a scientific perspective: In science we must clearly define the difference between SCALES and BALANCES.  A SCALE is a graduated range of values forming a standard system for measuring or grading something (e.g. the scale on a ruler).  A BALANCE is an instrument used to measure MASS (which leads us to the two next important terms).  MASS is a measure of the amount of matter that an object/substance contains (its inertial property).  WEIGHT is a measure of the force exerted on an object by gravity (the force needed to support it). Balances are instruments used to measure MASS (as opposed to WEIGHT). The MASS of an object does not change but its WEIGHT can change dependent on outside forces (i.e. gravity will affect weight). Think of it this way, a rhinoceros on Earth contains the exact same number of cells as would a rhinoceros on the moon; however, due to gravity, the mass of a rhino on Earth and the effect of gravity holds it planted firmly on the soil. That mass of that same rhino on the moon is not impacted by gravity in the same way and therefore is able to freely float around like a butterfly! The MASS is the same, the WEIGHT changed. As previously stated, you will be using BALANCES in this lab to measure the MASS of various objects. Balances can be divided into MECHANICAL (use moving parts – fulcrums, beams and springs) versus ELECTRONIC (few moving parts – use circuit boards and electronics). Balances can also be further divided into ROUGH or TOP- LOADING (sensitivity 1- 0.01g) and ANALYTICAL (sensitivity of 0.0001g or more). MATERIALS, EQUIPMENT AND REAGENTS: Various electronic balances Objects to measure (e.g. labelled glassware and metal calibrators) PROCEDURE: A variety of electronic balances will be made available to you, including both ROUGH/TOP-LOADING and ANALYTICAL balances. These are sensitive pieces of equipment that must be treated with care and therefore your instructor will demonstrate the correct way to use each type of balance. 1. Each student will measure (or mass) one (1) labelled piece of glassware and one (1) numbered calibration metal on each type of available balance. 2. Record your values in the table below. Label Rough/Top-Loading Balance Analytical Balance (±0.01g) (±0.0005g) Glassware #______ Metal #_______ ***NOTE: The instructor will have massed the items beforehand and will use those values for marking purposes. Reported values should be within the acceptable range (± 0.01g for top-loading rough balances and ± 0.0005g for the analytical balances). 1  CHEM 1001 Lab 2: Solutions and Dilutions PART II: PREPARING SOLUTIONS A solution is a homogeneous mixture (i.e. a mixture which has uniform composition and properties throughout; e.g. a teaspoonful of table salt stirred into a glass of water forms a homogeneous mixture) of two or more soluble chemical substances. The chemical that is dissolved (the lesser quantity) is called the SOLUTE and the chemical that is dissolving the solute (the greater quantity) is called the SOLVENT. The CONCENTRATION of a solution is an expression of the amount (usually MASS) of solute found in a given amount (usually VOLUME) of solvent. The mathematical calculation of concentration is often represented as follows: C = m/V (CONCENTRATION is equal to MASS divided by VOLUME) In this lab you will be learning how to safely prepare analytical level percent weight/volume concentration (which is written as [%(w/v)] ) solutions. Note: Square brackets [ ] are used to denote concentration. [%(w/v)] simply means the number of grams of SOLUTE in 100mL of SOLVENT (g/100mL). For example, a 1%(w/v) solution of sugar water would be made up by adding 1g of sugar to 100mL of water. In this lab each student will prepare the following aqueous dye solutions: Concentration Alternate Concentration Units Solution # Volume (mL) %(w/v) (g/100mL) Solution 1 0.10 100 0.10 Solution 2 0.1775 50 0.1775 Solution 3 0.0585 250 0.0585 MATERIALS, EQUIPMENT AND REAGENTS:  Analytical balances  Weigh boats  Scoopulas  Beakers  Glass stir rods  Funnels  Volumetric flasks and plastic stoppers  50 mL centrifuge tubes  Dye powder (this is your SOLUTE)  Squeeze bottles of distilled water (this is your SOLVENT) **Note: Always acquire knowledge of the chemicals that you are using before preparing solutions. Along with basic safety issues, in subsequent labs you will see how different types of chemicals will alter how you go about preparing different solutions. 2  CHEM 1001 Lab 2: Solutions and Dilutions PROCEDURE: 1. As a class, your instructor will work through the calculations for the amounts of solute (dye) required for each of the volumes of aqueous solutions to obtain the specified concentrations. Solution A: 0.10 %(w/v) Solution B: 0.1775 %(w/v) Solution C: 0.0585 %(w/v) V = 100 mL V = 50 mL V = 250 mL C = 0.10 %(w/v) C = 0.1775 %(w/v) C = 0.0585 %(w/v) = 0.10 g/100mL = 0.1775 g/100mL = 0.0585 g/100mL m = C V m = C V m = C V 2. Rinse all of glassware with distilled water PRIOR to beginning. This ensures that no residual chemicals are left behind that could contaminate the solutions you are preparing. 3. Using an analytical balance, measure out the required amount of solute (dye) in a weigh boat (these are used to contain materials being massed on the balance) and transfer measured amount to an empty beaker (rinsing the weigh boat with the distilled water solvent). 4. Add approximately half of the required volume of solvent (distilled water) to the solute in the beaker directing it down the side of the beaker. You can measure using the gradients on the side of the beaker. 5. Stir with a clean glass rod until the solute appears completely dissolved (a homogeneous mixture). 6. Because you are making analytical level solutions, you will now transfer all the beaker contents to a volumetric flask. You will use the QUANTITATIVE transfer method meaning that you will thoroughly rinse the contents of the beaker as well as the residuals on the glass rod through a funnel into the volumetric flask using distilled water (your solvent). 7. You will now add the remainder of your total solvent (distilled water) directly to the volumetric flask until the meniscus reaches the bottom of the etched line on the neck of the flask. 8. Stopper the volumetric flask with an appropriately sized cap and mix by slow inversion (put your thumb on the cap and gently turn the flask upside down then right side up three times. 9. Transfer ~50mL of the aqueous solution into a 50 mL centrifuge tube. Be sure to properly label the tube with the solute (dye), the solvent (distilled water), the final concentration, the date and your name using permanent marker directly on the disposable tube. 10. Repeat this process for each solution to be prepared. These solutions will be used in next week’s lab. 3  CHEM 1001 Lab 2: Solutions and Dilutions WASTE DISPOSAL Liquid Waste Waste container in the fumehood Weigh Boats Dispose in the large biohazard container 4