1. Unit 5, Lesson 6 Measuring Concentration the Chemist’s Way: MOLARITY

2. Review: What is concentration? The amount of solute that dissolves in the solvent

3. Review Concentration can be measured many ways:  Unsat’d, sat’d, supersat’d  Percent volume  Percent mass  Molarity

4. Chemists use Molarity to measure concentration of solutions.

5. Q1: What is the molarity of a 5.00 liter solution that was made with 10.0 moles of KBr ? # of moles of solute Molarity = ---------------------- Liters of solution 10.0 moles of KBr Molarity = -------------------------- = 2.00 M 5.00 Liters of solution

6. Q2: What is the molarity of a 6.00 liter solution that was made with 3.0 moles of KBr ? # of moles of solute Molarity = ---------------------- Liters of solution 3.0 moles of KBr Molarity = -------------------------- = 0.5 M 6.00 Liters of solution

7. But how do you measure moles? Does a balance or scale measure in moles or grams? Grams So, how do you measure out moles?!? You have to convert from moles to grams. How do you do that? Using molar mass How do you do that? EASY!

8. How to calculate molar mass… Use the formula, look up the atomic weight for each element on the periodic table, then add everything together. For example, in iron (II) chloride, or FeCl 2, you have one atom of iron and two atoms of chlorine. The molar mass will be equal to (1 atom x 56 grams/mole Fe) + (2 atoms x 35.5 grams/mole of chlorine) = 127 grams/mole of iron (II) chloride

9. Your turn! Calculate the molar mass of HCl (1x 1.0 H) + (1x 35.5 Cl) = 36.5 grams/mol of HCl This means that 1 mole of HCl will weigh 36.5 grams. Try this … What would 2 moles of HCl weigh? 36.5 grams/mol of HCl x 2 = 73.0 grams/mol of HCl

10. Putting it all together The lab says to make a 6.0 HCl solution, what do you do? 6M means 6 moles per liter, So 6 moles of HCl 35.5 grams/mol of HCl x 6 = 213.0 grams/mol of HCl Weigh out 213.0 grams of HCl and mix with 1 liter of water!

11. Your turn again… The lab says to make a 2.0 M NaOH solution, what do you do?