C.6-C.7 In which you will learn about: Different concentration units Calculating concentrations

C.6 Solution Concentration The general terms saturated and unsaturated are not always adequate to describe the properties of solutions that contain different amounts of solute. A more precise description of the quantity of a solute in a solution is needed—a quantitative expression of concentration.

What is concentration? Solution concentration refers to how much solute is dissolved in a specific quantity of solvent or solution. You have already worked with one type of solution concentration expression: the water-solubility curves (mass of solute dissolved in mass of water)

Percent by Mass Another way to express concentration is with percent values For example, dissolving 5g of table salt in 95 g of water produces a 100g solution with a 5% concentration of salt by mass % by mass grams solute grams total solution % by mass =

ppm AND ppb For solutions containing considerably smaller quantities of solute (as are found in many environmental water samples, including those from the Snake River), concentration units of parts per million (ppm) are sometimes useful Just as percent (per cent) means “for every hundred,” per million means “for every million.” ppm = (mass solute/mass solvent) x 1 000 000 For very low concentrations, parts per billion (ppb) is often used. ppb = (mass solute/mass solvent) x 1 000 000 000

Sample Problem What is the concentration of a 1% salt solution expressed in ppm? ANSWER: Because 1% of 1 million is 10 000, a 1% salt solution is 10 000 ppm.

C.7 Describing Solution Concentrations Sample Problem 1: A common intravenous saline solution used in medical practice contains 4.5 g NaCl dissolved in 495.5 g sterilized distilled water. What is the concentration of this solution, expressed as percent by mass? % by mass = (4.5 g NaCl/500.0 g solution) x 100 = 0.90%

Sample Problem 2 One teaspoon of sucrose, with a mass of 10.0 g, is dissolved in 240.0g water. What is the concentration of the solution expressed as grams sucrose per 100 g solution? As percent sucrose by mass? Answer 1: This solution contains 10.0 g sucrose and 240.0g water; therefore its total mass is 250.0g. Because 100.0 g solution would contain 2/5 as much solution, it would also contain 2/5 as much solute, or 4.0 g sucrose. Thus, the concentration is 4.0 g sucrose per 100.0 g solution. Answer 2: % by mass = (10.0 g sucrose/250.0 g sol’n)x 100 = 4.0%

The MOST IMPORTANTConcentration of Solute The amount of solute in a solution is given by its concentration. Molarity ( M ) = moles solute liters of solution

Step 1: Calculate moles of NiCl2•6H2O PROBLEM: Dissolve 5.00 g of NiCl2•6 H2O in enough water to make 250 mL of solution. Calculate the Molarity. Step 1: Calculate moles of NiCl2•6H2O Step 2: Calculate Molarity [NiCl2•6 H2O ] = 0.0841 M

moles = M•V USING MOLARITY What mass of oxalic acid, H2C2O4, is required to make 250. mL of a 0.0500 M solution? moles = M•V Step 1: Change mL to L. 250 mL * 1L/1000mL = 0.250 L Step 2: Calculate. Moles = (0.0500 mol/L) (0.250 L) = 0.0125 moles Step 3: Convert moles to grams. (0.0125 mol)(90.00 g/mol) = 1.13 g

Preparing Solutions Weigh out a solid solute and dissolve in a given quantity of solvent. Dilute a concentrated solution to give one that is less concentrated.

Calculating Concentrations Dissolve 62.1 g (1.00 mol) of ethylene glycol in 250. g of H2O. Calculate the % by mass of ethylene glycol.

Learning Check A solution contains 15 g Na2CO3 and 235 g of H2O. What is the mass % of the solution? 1) 15% Na2CO3 2) 6.4% Na2CO3 3) 6.0% Na2CO3

LEARNING CHECK: Using mass % How many grams of NaCl are needed to prepare 250 g of a 10.0% (by mass) NaCl solution?

Learning Check How many grams of NaOH are required to prepare 400. mL of 3.0 M NaOH solution? 1) 12 g 2) 48 g 3) 300 g

Learning Check Determine the grams of solute needed to prepare these solutions: A) 0.289 liters of a 0.00300 M Cu(NO3)2 solution B) 16.00 milliliters of a 5.90-molar Pb(NO3)2 solution C) 508 mL of a 2.75 M sodium fluoride solution