Aqueous Reactions and Solution Stoichiometry

Solution Composition


Water possesses many unusual properties. One of the most important properties of water is its ability to dissolve a wide variety of substances. It may sound strange, but absolutely pure water can be considered corrosive due to its capacity to absorb other compounds and ions.

Solutions in which water is the dissolving medium are called aqueous solutions.

Limestone caves, for example, are formed by the dissolving action of water, and dissolved CO2, on solid Calcium Carbonate. The dissolved mineral is then deposited as stalagtites and stalagmites as the water evaporates:

CaCO3(s) + H2O(l) + CO2(aq) - Ca(HCO3)2(aq)

Many physiological chemical reactions occur in aqueous solutions.

How do we express solution composition?

What are the chemical forms in which substances occur in aqueous solutions?
 

Solution Composition

A solution is a homogenous mixture of two or more substances, consisting of

  1. The solvent - usually the substance in greater concentration
  2. The other component(s) is (are) called the solute(s) - they are said to be dissolved in the solvent

When a small amount of NaCl is dissolved in a large quantity of water, we refer to the water as the solvent and the NaCl as the solute.

Molarity

The term concentration is used to indicate the amount of solute dissolved in a given quantity of solvent or solution.

The most widely used way of quantifying concentration in chemistry is molarity.

The molarity (symbol M) of a solution is defined as the number of moles of solute in a liter volume of solution:

For example, a 1.0 molar solution (1.0 M) contains 1.00 mol of solute in every liter of solution.
 

What is the molarity of a solution made by dissolving 20 grams of NaCl in 100 mls of water?

M solution

If we know the molarity of a solution we can calculate the number of moles of solute in a given volume. Thus, molarity is a conversion factor between volume of solution and moles of solute:
 

Calculate the number of moles of CaCl2 in 0.78 liters of a 3.5 M solution:

CaCl2

How many liters of a 2.0 M solution of HNO3 do we need to have 5 moles of HNO3?

Note: we had to invert the stock solution (i.e. convert to liters per mole) to be able to calculate the needed volume (i.e. to keep the dimensional analysis correct)
 

Dilution

For convenience, solutions are either purchased or prepared in concentrated stock solutions which must be diluted prior to use.

When we take a sample of a stock solution we have a certain number of moles of molecules in that sample. Dilution alters the molarity (i.e. concentration) of the solution but not the total number of moles of molecules in the solution (in other words, dilution does not create or destroy molecules).

One of the standard equations for determining the effects of dilution upon a sample is to set up an equation comparing (concentration)*(volume) before and after dilution. Since (concentration)*(volume) gives us the total number of moles in the sample, and since this does not change, this value before and after dilution are equal:

(concentration)*(volume) = (concentration)*(volume)

(moles/liter)*(liter) = (moles/liter)*(liter)

moles = moles

How much of a 5 M stock solution of NaCl will you need to make up 250 mls of a 1.5 M solution?

X liters = 0.075 liters (or 75 mls)

Thus, we would need 0.075 liters of our 5M NaCl stock solution. The rest of the 0.25 liter volume is made up by the addition of water:

0.25 liters - 0.075 liters = 0.175 liters

So we would take 0.075 liters of stock 5M NaCl solution and add to that 0.175 liters of water for a final volume of 0.25 liters with a final concentration of 1.5 moles/liter (i.e. 1.5 M)


What is the concentration of water?

Molecular weight of H2O = 18.0g/mole

Density of H2O = 1g/ml or 1000g/L

Pure water is 55.6M H2O


1996 Michael Blaber