SOLUTIONS

A solution is a special type of mixture that is homogeneous throughout. This means that the molecules or ions involved are so well mixed that the composition is uniform throughout the mixture. Think of salt-water. You cannot see salt within the water when it is fully dissolved, not even with the aid of a microscope. (This contrasts with a heterogeneous mixture in which you can identify the separate components. For example, a mixture of salt and sand is heterogeneous.)
A solvent is the component in a solution that is present in the largest amount. In a NaCl solution (salt-water), the solvent is water.A solute is the component in a solution in the lesser amount. In a NaCl solution, the salt is the solute. A solution may contain more than one solute.
There are different types of solutions. The one you are probably most familiar with is the aqueous solution.An aqueous solution is a solution in which water is the solvent. A NaCl solution is an aqueous solution.A non-aqueous solution is a solution in which water is not the solvent. Examples of non-aqueous solutions are solutions used in dry cleaning (a solution of ethene in the solvent dichloromethane).A solid solution is a solution in which a solid is the solvent. An example is a brass solution that is formed by dissolving copper in zinc.
So what happens when you drop salt into a glass of water? The water before and after does not look different (assuming that all of the salt is dissolved). However, if you took a drink of it, it certainly tastes different. It boils at a higher temperature than pure water and it conducts electricity. What happened?

Your everyday table salt consists of NaCl. Water is made of H2O molecules. When these two are combined together in a solution, NaCl actually separates into ions. [In solid NaCl, Na+ and Cl- ions are arranged in an ordered three dimensional array called a crystal lattice, as depicted in the figure below.] Thus NaCl (s) becomes Na+ and Cl- ions in solution (i.e., NaCl dissolves in water). Why does NaCl dissolve?
Even though NaCl dissolves to become ions in a solvent, the overall charge remains neutral. Remember that it is NaCl, a neutral compound, that forms the Na+ and Cl- ions. There will be an equal number of positive and negative charges; therefore, the solution will be neutral.

Concentrations of Solutions

There are a number of ways to express the relative amounts of solute and solvent in a solution.
Percent Composition (by mass)

We can consider percent by mass (or weight percent, as it is sometimes called) in two ways:
The parts of solute per 100 parts of solution.
The fraction of a solute in a solution multiplied by 100.
We need two pieces of information to calculate the percent by mass of a solute in a solution:
The mass of the solute in the solution.
The mass of the solution.

Molarity
Molarity tells us the number of moles of solute in exactly one liter of a solution. (Note that molarity is spelled with an "r" and is represented by a capital M.)
We need two pieces of information to calculate the molarity of a solute in a solution:
The moles of solute present in the solution.
The volume of solution (in liters) containing the solute.

Molality

Molality, m, tells us the number of moles of solute dissolved in exactly one kilogram of solvent. (Note that molality is spelled with two "l"'s and represented by a lower case m.)
We need two pieces of information to calculate the molality of a solute in a solution:
The moles of solute present in the solution.
The mass of solvent (in kilograms) in the solution.

Mole Fraction

The mole fraction, X, of a component in a solution is the ratio of the number of moles of that component to the total number of moles of all components in the solution.
The number of moles of each component present in the solution.

MASS PERCENT CONCENTRATION PROBLEMS

1.) A 4 g sugar cube (Sucrose: C12H22O11) is dissolved in a 350 ml teacup of 80 °C water. What is the percent composition by mass of the sugar solution?
Given: Density of water at 80 °C = 0.975 g/ml.

2.) Concentrated sulfuric acid is 96.0% H2SO4 by mass. (The remaining 4% is water.) The density of this solution is 1.84 g/cm3. Calculate the number of moles of H2SO4 in a liter of concentrated sulfuric acid

3) What is the weight percent of glucose in a solution made by dissolving 4.6 g of glucose in 145.2 g of water?

4) How would you prepare 400. g of a 2.50% solution of sodium chloride?

5) What is the weight percent of ethanol in a solution made by dissolving 5.3 g of ethanol in 85.0 g of water?

6) How would you make 250. g of a 7.5% solution of glucose in water?

7) A sample of a solution weighing 850.0 g is known to contain .223 moles of potassium chloride. What is the weight percentof potassium chloride in the solution.

MOLARITY PROBLEMS

1.) What is the molarity of a solution made by dissolving 2.5 g of NaCl in enough water to make 125 ml of solution?

2.) How would you prepare 400.0 ml of 1.20 M solution of sodium chloride?

3.) How would you prepare 100.0 mL of 0.25 M KNO3 solution?

4.) A chemist dissolves 98.4 g of FeSO4 in enough water to make 2.000 L of solution. What is the molarity of the solution?

5.) How many moles of KBr are in 25.0 mL of a 1.23 M KBr solution?

6.) Battery acid is generally 3 M H2SO4. Roughly how many grams of H2SO4 are in 400.0 mL of this solution?

7.) Calcuate the molarity when 75.0 grams of MgCl2 is dissolved in 500.0 mL of solution.

8.) How many grams of KMnO4 are needed to make 600.0 mL of a 0.200 M solution?

MOLALITY /MOLARITY/MOLE FRACTION PROBLEMS

1.) Calcuate the molality when 75.0 grams of MgCl2 is dissolved in 500.0 g of solvent.

2.) What is the molality of a solution made by dissolving 5.0g of toluene (C7H8) in 225g of benzene (C6H6)?

3.) The concentrated sulfuric acid we use in the laboratory is 98.0% H2SO4 by mass. Calculate the molality and molarity of the acid solution. The density of the solution is 0.982g/mL.

4.) If you want a solution that is 0.100m in ions, how many grams of Na2SO4 must you dissolve in 125g of water? (assume total dissociation of the ionic solid)

5.) You want to prepare a solution that is 0.200m in Na2CO3. How many grams of the salt must you add to 125g of water? . What is the mole fraction of Na2CO3 in the resulting solution?

6.) A solution is prepared by dissolving 34.2 g of MgCl2 in 0.430 L of H2O. Calculate the molarity, molality and mole fraction of MgCl2 if the density of water is 1.00 g/cm3 and the density of the solution is 1.089 g/cm3.


STUDENTS:

Select 3 problems in each type of concentration problems.
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