Concentration expressed as Molarity:

May 07, 2013Concentration expressed as Molarity:Let’s review the symbols in a chemical equationCaCO 3(s)+ 2HCl (aq)CO 2(g) +CaCl 2(aq)+ H 2O (l )CaCO 3Calcium carbonate is a solid. CO 2(g) is a gas H 2O (l )is a liquidHCl (aq)and CaCl 2(aq)are aqueous solutions This means that the hydrochloric acid and the Calcium chloride aredissolved in water (aqueous),The unit of measure for aqueous solutions is MolarityM, which stands for ( moles/liter) moles per liter of solutionMolarity is a measure of the concentration of a solutionQuick review of dissolving: Solvent:Solute:The dissolving liquid – typically waterThe substance being dissolved . For example: salt, sugar,acids,Solution: The combination of solvent and soluteConcentration: The amount of solute dissolved in a specific amount of solution(how much solute packed into a given volume)Solutions can be dilute low concentration Solutions can be concentrated :Low molarityhigh molarity0.5 M3 MNote: M molarity is moles per liter of solution not moles per liter of solventWhen we make a liter of 3 M NaCl solution, we do not combine 3 moles of NaCl with 1 liter of water.If we combine 3 moles of NaCl with one liter of water, we end up with more than a liter of solution. The3 moles of NaCl takes of space. It adds to the volume.To make a 3M solution combine 3 moles of NaCl and fill with enough water to make 1 liter of solution(the total combination of salt and water equals 1 liter of solutionThe concentration of a solution is measured in molarity The units are :moles/liter or M3 M solution “3 molar solution” means 3 moles of solute in every liter solution.A 3M NaCl solution means that there are 3 moles of NaCl in every liter of NaCl and water solutionIn order to make a liter of 3M NaCl, we would place 3 moles of NaCl into a large beakerand then fill with distilled water until it reaches 1 liter.1 mole NaCl = 23.00 + 35.5 = 58.5g3 moles NaCl x 58.5 g = 175.5 grams of NaCl1 mole NaClSo we’d add 175.5 g of NaCl to the beaker and then fill the rest of the way to 1 liter of solution

May 07, 2013If we took this 1 liter of 3M solution and divided it up into different beakers, they would all still have a concentration of 3M solution.Molartiy refers to concentration . We can have a teaspoon of 3M solution, we can have 100 ml of a 3M solution, we can have a gallon of a 3Msolution.All of them have a concentration of 3 moles per liter3M3MSample Problem 1: How many moles of NaCl are there in 100 ml of 3M NaCl solution?100ml x 1 liter x 3 moles = 0.3 moles NaCl1000ml 1 liter"sweetness" or "saltiness"has not changed2 types of problems: 1. How many moles are in the solution 2. How many ml or liters would we need toget a certain number of moles.And of course if you wanted to know how many grams of NaCl you’d need simply multiplyby g/mole from the periodic table0.3 moles NaCl x 58.5 g = 17.55 g1 moleStrategy: When given a Molarity problem always rewrite the M as moles over 1 liter mole1 literIf the problem asks for moles or grams, and gives you a 3 M solution rewrite the 3M as 3 moles1 literand multiply by volume. The liters will cancel and you’ll end up in molesSample Problem 2: If I wanted 0.5 moles of HCl, How many ml of a 4M HCl solution would I need to get 0.5 molesof HCl?0.5 moles x 1 liter = 0.125 liters but I want ml so 0.125 liters x 1000 ml = 125 ml4 moles 1 literStrategy: Rewrite the 4M as 4 moles Since the problem asks for volume, flip the 4M so that liters are on top1 liter4 moles becomes 1 liter1 liter 4 molesNow just multiply by the moles given in the question. Moles will cancel and you’ll be in liters! (seee above)

May 07, 2013Molarity also helps when doing stoichiometry. For example: Mg(s) + 2HCl (aq)Problem: How many grams of magnesium would we need to react 100 ml of 3M HCl?MgCl 2(aq)+ H 2(g)Strateg y: Start with what you know and find a way to moles.Convert to liters, convert to moles, equation ratio for moles Mg, convert to grams Mg100 ml x 1liter x 3mole s x 1Mg x 24.3 g =1000ml 1 liter 2HCl 1 moleSample Problem 2: Mg (s)+ 2HCl (aq)MgCl 2(aq)+ H 2(g)If I began with 15 grams of Mg, how many ml of 5 M HCl would I need?2 Al (s)+ 2 KOH (aq)+ 6 H 2O (l)2 KAl(OH) 4 (aq)+ 3 H 2 (g)A. If I wish to react 120 grams of Aluminum, how many ml of 6 M KOH will I need?B. If I react 1500 ml of 3 M KOH, how many grams of H2O will I need?4NH 3(aq)+ 5O 2(g)4NO (g)+ 6H 2O (l)1. If I began with 350 ml of 3M NH3, how many grams of O 2would I need?2. If I wish to produce 280 grams of H 2O, how many ml of 5M NH 3would I need to react?

May 07, 2013Molarity Practice Problems: Show all work, show all units1. How many moles are in 3 liters of a 5M solution?2. How many moles are in 250 ml of a 3 M solution?3. How many grams of NaF are in 600 ml of a 2M solution?4. How many grams of CaI 2are in a 2400 ml of a 3M solution?5. How many liters of a 2 M solution would I need to obtain 6 moles?6. How many ml of a 4 M solution would I need to obtain 7.5 moles?7. How many ml of 3 M solution would I need to obtain 0.5 moles?8. How many ml of a 5 M solution would I need to obtain 3.5 moles?9. Describe how you would prepare 1 liter of a 2 M NaOH solution. (how many grams of NaOH would you need and howmuch water you would add)10. Describe how you would prepare 2 liters of a 2M NaOH solution. (How many grams of NaOH would you need andhow much water would you add)11. Describe how you would prepare 500 ml of a 2M NaOH solution.12. When looking at the equation: CaCO3(s) + 2HCl (aq)CO 2(g) +CaCl 2(aq)+ H 2O (l )What does the (s) stand for? What does the (g) stand for? What does the (l) stand for?What does (aq) stand for and what units would you use to measure it?13. Please define the following and give an example: A. Solute B. solvent C. solution D. concentration14. What does the symbol M stand for and what does it mean?