1. Composition of Substances and Solutions
Chapter 3

2. Objectives 2.0 Define key terms and concepts.
2.1 Calculate the number of molecules of a compound or element.
2.2 Calculate the percent concentration for each element in a compound.
2.3 Determine the empirical molecular for a compound given the amount of each element present.
2.4 Calculate the concentration of a solution.
2.5 Prepare a dilute solution from a more concentrated solution.
2.6 Convert between grams and moles.

3. Avogadro's Number and The Mole
The unit by which chemists count atoms and molecules.
Indicated in formulas by the letter n.
Avogadro’s Number
Number of molecules in one mole of a substance.
6.02 x 1023 molecules = 1 mole

4. Avogadro's Number and The Mole

5. Molar Mass The mass of one mole of a substance.
Obtained by adding together the molar masses of the elements present.
Can be used to convert between moles and grams of a substance.

6. Calculate the molar mass for the following compounds.
NaCl
MgBr2
H3PO4
Ca(NO3) 2
N2O2

7. Molar Conversions Mass of Element ÷ Molar Mass
Moles x Avogadro's Number
Mass of Element
Moles of Element
Atoms of Element
Moles x Molar Mass
Molecules ÷ Avogadro's Number

8. How many moles of Mg(NO3)2 are present if you have 54
How many moles of Mg(NO3)2 are present if you have 54.36g of the compound? How many molecules of the compound are present?

9. How many molecules of CaBr2 are present if you have 4
How many molecules of CaBr2 are present if you have 4.9g of the compound?

10. Percent Composition Mass of each element in a compound.
Can be used to help determine the purity of a compound after synthesis.
n x molar mass of element
Molar mass of compound
Percent Composition =
x 100

11. Calculate the percent composition for each of the elements CaCl2.

12. Calculate the percent composition for each of the elements in Mg(NO3)2.

13. Empirical Formula
The simplest whole number ratio of atoms of each element present in a compound.

14. Steps for Determining an Empirical Formula
1. Start with the number of grams of each element, given in the problem. 
If percentages are given, assume that the total mass is 100 grams so that the mass of each element = the percent given.
2. Convert the mass of each element to moles.
3. Divide each mole value by the smallest number of moles calculated. 
4. Round to the nearest whole number.  This is the mole ratio of the elements and is represented by subscripts in the empirical formula. 
If the number is too far to round (x.1 ~ x.9), then multiply each solution by the same factor to get the lowest whole number multiple. 
e.g.  If one solution is 1.5, then multiply each solution in the problem by 2 to get 3. 

15. A compound was analyzed and found to contain 13. 5 g Ca, 10
A compound was analyzed and found to contain 13.5 g Ca, 10.8 g O, and g H.  What is the empirical formula of the compound?

16. Acrylic Acid is used the manufacturing of plastics
Acrylic Acid is used the manufacturing of plastics. It’s composition is 50.5% C, 5.6% H, and 44.4% O by mass. What is its empirical formula?

17. NutraSweet is 57. 14% C, 6. 16% H, 9. 52% N, and 27. 18% O
NutraSweet is 57.14% C, 6.16% H, 9.52% N, and 27.18% O.  Calculate the empirical formula of NutraSweet and find the molecular formula.  (The molar mass of NutraSweet is g/mol)

18. Two compounds have the same composition: 85. 62% C and 14. 38% H
Two compounds have the same composition: 85.62% C and 14.38% H. What is the empirical formula for these compounds? What are the molecular formulas if the compounds have molar masses of amu and 28.03amu?

19. What Are Your Questions?

20. Concentration of a Solution

21. Solubility
As the temperature of a solution increases, its solubility increases.
The solubility of a gas decreases with increased temperature
The solubility of a gas increases with increased pressure

22. Solutions Electrolytes Nonelectrolyte
Separate into ions when dissolved in water (ionic compounds and compounds with polyatomic ions)
Conducts electricity
Nonelectrolyte
Do not separate into ions when dissolved in water
Do not conduct electricity

23. Solutions Strong Electrolytes Dissociate
Dissociates completely when it dissolved in water
Dissociate
When a molecule separates into ions

24. Solutions
Hydration
When water molecules surround an ion in a specific manner

25. Solutions Weak Electrolytes Chemical Equilibrium
Solute dissolve in water mostly as whole molecules (does not dissociate)
Chemical Equilibrium
The point in a chemical reaction where molecules dissociate as fast as they recombine and no net- change can be observed.

26. Solutions Type of Solutes Dissociation Contained in Solution
Conducts Electricity
Examples
Strong Electrolytes
Completely
Only Ions
Yes
Ionic Compounds, strong acids and bases
Weak Electrolyte
Partially
Mostly Molecules with a few ions
Yes, but not well
Weak acids and bases such as HF, CH3OOH, H2O, NH3
Nonelectrolyte
None
Only Molecules No
Carbon Compounds such as alcohols and sugars

27. Concentration of a Solution
Molarity (M)
The amount of solute in moles divided by the amount of solvent measured in Liters
Another way to measure concentration
Measured in moles per Liter (mol/L)
Moles
Molarity x Volume

28. What is the molarity of a solution containing 3
What is the molarity of a solution containing 3.2 moles of NaCl dissolved in 0.56L of solute.

29. If 7.9g of KI are dissolved in 500mL of water, what is the concentration of the solution?

30. How many grams of copper (II) nitrate need to be dissolved in 1
How many grams of copper (II) nitrate need to be dissolved in 1.5L of water to produce a 0.25M solution?

31. Dilution
Making a less concentrated solution from a more concentrated one.
CiVi = CfVf
What volume of solution would you need to prepare 500mL a 0.35M solution if you started with a 1.2M solution of HCl?

32. If 52mL of a 2.5M KCl solution is diluted to a volume of 500mL, what is the concentration of the new solution? How many grams of KCl are present?

33. You have 107mL of a 0.125M solution of HNO3 and you want to dilute that entire amount to a 0.075M solution. How much water should you add to the solution?

34. Percent Concentration

35. What is the percent by mass of 8.1g of solution containing 2.7g KI?
What is the percent by mass of a solution containing 100g of water and 10g of HCl?

36. If 35mL of HCl are dissolved in 250mL of solution, what is the percent by volume of HCl present?
What is the percent by volume of a 300mL solution containing 239mL ethanol?

37. 500g aqueous solution is 21% sodium chloride
500g aqueous solution is 21% sodium chloride. How many grams of solute and solvent are present in the solution?

38. A laboratory procedure requires you to use 250mL of a 30% hydrochloric acid solution. Calculate how many mL of HCl are present in this solution?

39. A bottle of beer is 6. 2% by volume ethanol, C2H5OH
A bottle of beer is 6.2% by volume ethanol, C2H5OH. How many moles of ethanol are present in a 12 oz (355mL) bottle of beer? The density of ethanol is 0.789g/mL.

40. What Are Your Questions?