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Understanding Molarity and Molality in Chemistry

Learn about molarity and molality in chemistry, calculating concentrations using moles, liters, grams, and dimensional analysis. Practice problems included.

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Understanding Molarity and Molality in Chemistry

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  1. BELLWORK 2/26/2015

  2. ANSWER – B

  3. Molarity and Molality Chapter 12

  4. Molarity • Chemists describe solutions in terms of concentration. We define the concentration of a solution as the amount of solute in a given volume of solution. • The most commonly used expression of concentration is molarity (M). • Molarity is defined as the number of moles of solute dissolved in a liter of solution. • A 1.0 M solution contains 1.0 mol of solute per liter of solution

  5. Molarity – (M) • Molarity is calculated using the following equation: • What does this mean for you? • Set-up these problems using dimensional analysis • If you’re given grams, you have to convert grams to moles • If you’re given mL, you have to convert mL to L ( 1L =1000mL) M (molarity) = moles of solute M= mol liters of solution L

  6. Molarity Example 1 • 5.7 g KNO3dissolves in a 233 mL solution. What is molarity? • Convert grams to moles • Convert mL to L • Divide mol/L 5.7 g KNO3 1 mol KNO3 = 0.056 mol KNO3 101.10 g KNO3 233 mL 1 L = 0.233 L 1000 mL 0.056 mol KNO3 0.233 L = 0.24 M KNO3

  7. Molarity Example 2 • Calculate the molarity of a solution that has 11.5 g of NaOH dissolved in 1500 mL of solution • Convert grams to moles • Convert mL to L • Divide mol/L 11.5 g NaOH 1 mol NaOH= 0.288 mol NaOH 40.0 g NaOH 1500 mL 1 L = 1.5 L 1000 mL 0.288 mol NaOH 1.5 L = 0.19 M NaOH

  8. Molality Molality(m) = Moles of Solute / Kg of Solvent

  9. Molality Example Example #1 – Suppose you had 2.00 moles of solute dissolved into 1.00 L of solvent. What's the molality?

  10. ANSWER – 2.00 moles 1L = 1kg

  11. EXAMPLE #2 What is the molality when 0.75 mol is dissolved in 2.50 L of solvent?

  12. ANSWER - 0.30 moles 1L = 1kg 2.50 L = 2.50 kg

  13. Now You Try… 3) Calculate the molality of 25.0 grams of KBr dissolved in 750.0 mL pure water.

  14. ANSWER 3 • Convert 25 g of KBr to moles : 25g x 1 mole = .210 mol • 119.0 g • Convert mL to L: 750 mL x 1L = .750 L • 1000mL • Convert L to kg: .750 L = .750kg • Divide moles /kg: .210 mol • .750 kg • 5. molality = .280 m (

  15. CLASSWORK / HOMEWORK:CH.12 WKSHT #2

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