SOLUTIONS AND SOLUBILITY

Transcription

1 Molarity is simply a measure of the "strength" of a solution. A solution that we would call "strong" would have a higher molarity than one that we would call "weak". The unit for molarity is M and is read as "molar". (i.e. 3 M = three molar). It can also be written as moles/liter. The formula is found on Table T of your reference tables. BASIC MOLARITY PROBLEMS WHERE THE MOLARITY IS UNKNOWN Example : What is the molarity of a 5.00 liter solution that was made with 10.0 moles of KBr? SOLUTIONS AND SOLUBILITY 2. Fill in the knowns - Molarity = 10.0 moles of KBr = 2.00 M 5.0 Example: A 250 ml solution is made with 0.50 moles of NaCl. What is the Molarity of the solution? 2. DON T Get FOOLED by the ml, convert to L! 250 ml 1 L X = 0.25 L 1000 ml 0.50 moles 3. Fill in the knowns - Molarity = 0.25 = 2.00 M BASIC MOLARITY PROBLEMS WHERE THE VOLUME IS UNKNOWN Example A : What would be the volume of a 2.00 M solution made with 6.00 moles of LiF? 2. Then re-arrange to isolate our unknown - Liters = Molarity 3. Fill in the knowns - Liters = 6.00 mol LiF 2.00 M = 3.00 L of solution 1 Molarity Cheat Sheet and Problems

2 Example B : What is the volume of 3.0 M solution of NaCl made with 526 g of solute? 2. Then re-arrange to isolate our unknown - Liters = Molarity 3. Figure out how many moles are in 526g of NaCl - Use the MOLE CALCULATION from your reference tables! given mass (g) = 526 g = 8.99 mol number of moles = gram-formula mass 58.5 g / mol 4. Fill in the knowns - Liters = 8.99 moles NaCl = 3.00 L of solution 3.0 moles/ L PRACTICE PROBLEMS 1. How many grams of CaCl 2 would be required to produce a 3.5 M solution with a volume of 2.0 L? 2. What is the molarity of a 500 ml solution containing 249 g of KI? 3. How many moles of LiF would be required to produce a 2.5 M solution with a volume of 1.5L? 2 Molarity Cheat Sheet and Problems

3 4. Find the molarity (M) of a solution that has 1.5 moles of solute in 2 L of solution. 5. Find the molarity of a solution that contains 6.0 moles of NaNO 3 in 250 ml of solution? 6. Find the molarity of a solution that contains 235 grams of KI in 4 L of solution. 7. HOW MANY MOLES OF SOLUTE ARE REQUIRED TO MAKE: A) 100mL of a 6 M solution B) 3 liters of a 2.5 M solution C) 250 ml of a 6 M solution D) 500 ml of a 3 M solution E) 4 Liters of a 1.5 M solution F) 1.5 L of a 1 M solution 3 Molarity Cheat Sheet and Problems

4 8. HOW MANY GRAMS OF SOLUTE ARE NEEDED TO PREPARE: A) 2 liters of a 1.5 M KOH B) 500 ml of a 6 M HCl C) 250 ml of a 4 M KNO 3 D) 100 ml of a 3 M KBr E) 4 L of a 0.5 M LiF F) 400 ml of a 1 M CaCl 2 9. FIND THE MOLARITY (how many moles there would be in 1 liter) OF A SOLUTION THAT HAS: A) 4 moles of solute in 2 L of solution B) 3 moles of solute in 500 ml of solution C) 0.5 moles of solute in 200 ml of solution D) 4 moles of solute in 250 ml of solution E) 0.2 moles of solute in 1 L of solution 4 Molarity Cheat Sheet and Problems

5 10. FIND THE MOLARITY OF A SOLUTION THAT CONTAINS: A) 505 g KNO3 in 500 ml of solution B) 20 g NaOH in 2000 ml of solution C) 80 g NaOH in 250 ml of solution D) 60 g NaOH in 1000 ml of solution E)40 g NaOH in 500 ml of solution F) 272 g CaSO 4 in 500 ml of solution G) 72 g Mg(NO 3) 2 in 250 ml of solution H) 400 g CaBr 2 in ml of solution i) 56 g CaO in 100 ml of solution J) 23 g Li 2S in 200 ml of solution 5 Molarity Cheat Sheet and Problems

6 Another way to express Concentration [ ] is to use Parts Per Million (ppm). This is used when the concentration of the solution is very dilute. Scientists usually express the maximum allowable concentrations of toxic or cancer-causing substances as ppm. The formula is found on Table T of your reference tables. IMPORTANT: The numerator and denominator must be in the same units of mass. Example: A CuSO4 solution contains g of of CuSO4 in 1000 g of solution. Find the PPM. parts per million =! parts per million =! g 2. Fill in the knowns - PPM = 1000 g X = 50 ppm Example: A solution has 300 ppm of KOH, find the number of grams of KOH in 1000 grams of this solution parts per million =! Fill in the knowns ppm = X g X = 0.3 g 1000 g These are 2 other ways to describe concentration: PERCENT BY VOLUME - Used to describe the [ ] of a solid or liquid in a solution. For a solid solute : % by volume = X 100 ml of solution For a liquid solute: ml of % by volume = solute X 100 ml of solution PERCENT BY MASS - Used to describe the [ ] of a solid or liquid in a solution. % by mass = X Molarity Cheat Sheet and Problems