Solutions and Solubility
1. Solutions
2. Molarity
3. Molality formula
4. Dilutions formula
5. Solubility
Solutions
What is a solution?
A solution is a homogeneous mixture in which one substance is dissolved in another substance.
For example:
In this example the substance being dissolved is salt and it is being dissolved in water.
There are three factors that affect the rate of solutions (how fast they dissolve) and affect solubility:
1. Stirring
2. Particle size (the smaller the better, it covers more surface area)
3. Thermal energy (heat)
Temperature (thermal energy) is different for gases. Unlike liquid and solids, gases become less soluble as the temperature increases.
Molarity
Molarity represents the number of moles of solute per liter of solution.
moles
M = -------
liters
i. How many grams of CaCl2 are in 350 mL of 2.0 M CaCl2 solution?
First you have to figure out what the moles are by using the formula for molarity (above).
So the equation would be moles= Molarity x Liters
plug in the numbers and solve:
m = 2 x 0.35 (you convert 350 ml to liters, so you get 0.35 L)
m = 0.7
Now that you know moles you can solve for grams by using the formula g = moles x mm (molar mass).
To find molar mass of CaCl2 use the periodic table. It will end up being 110.984.
Plug in the numbers and solve:
g = 0.7 x 110.984
g = 77.689 That's the answer!
ii.What is the molarity of 2.5 L of solution that contains 1.0 mol MgCl2?
To solve for molarity plug the numbers into the equation and solve:
M = 1.0 / 2.5 L
M = 0.4 M That's the answer!
iii. How many moles of solute are present in 400 mL of 2.0 M KNO3?
To solve for moles use the molarity formula to solve for moles, so the equation would be m = M x L.
Plug in the numbers and solve:
m = 2 x 0.4 (convert 400 ml to liters, so that it's 0.4 L)
m = 0.8 m That's the answer!
Molality formula
Molality represents the number of moles of solute per kilogram of solvent.
moles
m = ----------
kilograms
i. Calculate the molality of a solution that has 1.5 moles added to 675 g of solvent.
To solve for molality use the formula above to solve.
Plug in the numbers and solve:
m = 1.5 / 0.675 (convert 675 g to kg by dividing by 1,000 to get 0.675 kg)
m = 2.22 m That's the answer!
Dilutions formula
The formula is M1 x V1 = M2 x V2
i. If you dilute 70.0 mL of 5.0 M NaCl to make a 350.0 mL of solution, what is the molarity
of the dilute solution?
Use the formula above to solve:
M1 x V1 = M2 x V2
5.0 x 70 = M x 350
350 = M x 350 Divide 350 on both sides
1 = M That's the answer!
ii.If you had 40.0 mL of a 3.00 M hydrochloric acid solution and you wanted to dilute it to
2.00 M, what would be the new volume?
Use the formula above to solve:
M1 x V1 = M2 x V2
3 x 40 = 2 x V
120 = 2 x V Divide 2 on both sides
60 = V That's the answer!
Solubility
The solubility rules are:
i. How much KNO3 will dissolve in 50 mL of water that is at 30 °C? ___180_g_______
ii. If you stir 20 grams of KNO3 in 100 mL of water that is at 50°C, will all of it dissolve?
___yes____
iii. A solution that contains 120 grams of MgSO4 in 100 mL of water at 80°C would
be saturated, unsaturated, or supersaturated? _____supersaturated__________
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