Solutions - CHEM 1305 - Texas State Technical College - Chemical Technology

PHARMACEUTICAL MATH I

PHRA 1209

Solutions

Problems

A solution is a homogeneous mixture of two or more substances.

Some types of solutions are familiar. For example, salt solution is salt dissolved in water. A gaseous solution we all use is air which is a solution of various gases like oxygen, nitrogen, carbon dioxide, water and other gases. It is possible to have solid solutions. Metal alloys are solid solution. An example is brass which is a solution of copper and zinc. Two liquids may dissolve to give a liquid solution. Examples include gasoline, alcoholic beverages, vinegar, bleach etc.

A solution is generally composed of two substances.

The solute is the substance being dissolved and is in smaller quantity.

The solvent is the substance that dissolves the solute and is in larger quantity.

Properties of Solution

Solutions are homogeneous mixtures. The properties are uniform throughout the mixture.
In a solution, proportions of solute and solvent can be varied. That is, the amount of solute and solvent can be changed.
The solute is not visible to the unaided eye. This is because the solute breaks down to ionic or molecular size.
The solute does not settle out of solution.
The solute can not be filtered out of solution.
The solute can be separated from solution by changing temperature. That is, by freezing or boiling, or evaporation.

Factors That Affect Solubility

What determines if a solute will dissolve in a given solvent?

The nature of solute and solvent. " LIKE DISSOLVES LIKE". Water dissolves salts, but not oil or gasoline.

Temperature: the higher the temperature, more solute will dissolve in a given solvent.

Pressure: pressure has little or no effect on the solubility of a solid or liquid, but the solubility of gases is greatly affected by pressure (Henry's Law).

Henry's Law

Henry's Law - the solubility of a sas in a liquid is directly proportional to the applied pressure.

This means that higher the pressure, the more gas dissolves in a liquid.

Carbonated soft drinks are examples of a gas (carbon dioxide) dissolved in a liquid (water) at high pressure. When a soft drink can is opened, the dissolved carbon dioxide escapes ( bubbles). If this can is left open for some time, the drink goes "flat". All of the carbon dioxide has escaped.

What can be done to increase the rate at which a solute dissolves in a solvent?

Factors That Affect the Rate of Dissolution

Particle size: the smaller the particle size, the faster the rate of dissolution. Powders dissolve faster than large lumps.
Temperature: the higher the temperature, the faster is the rate of dissolution. It is easier to dissolve sugar in a glass of hot tea than it is in a glass of iced tea.
Rate of stirring: stirring or agitation of the solute in a solvent causes it to dissolve faster.
Concentration: if a solution has some solute dissolved in it, the rate of dissolving additional solute will be slower.

Relative Terms for Expressing Solute Concentration

Solubility is the amount of solute that can be dissolved in a given amount of solvent.

Non-quantitaive terms: this means it does not contain any numbers but is based on relative strengths.

Concentrated solution: the solution contains more solute than a dilute solution.

Dilute solution: the solution contains less solute than concentrated solution.

Unsaturated solution: the solution can dissolve more solute.

Saturated solution: the solution contains the maximum amount of solute the solvent can dissolve.

Supersaturated solution: the solution contains more solute than a saturated solution. This is very unstable condition and slight disturbance causes the excess solute to settle out.

Note: All of these are temperature dependent. A solution that is saturated at 25^oC may be unsaturated at 45^oC.

Quantitative Terms for Expressing Solute Concentration

Solutions are made up of solute and solvent. Concentration terms are basically a relationship between the amount of solute present and the amount of solution.

Percent Concentration of Solute

The concentration of a solution can be expressed as the percentage of the solute based on the entire solution. The solution is the total of the solute and the solvent.

The concentration can be expressed as the percent by mass.

Example: 20 grams of NaCl is is mixed with 180 ml of water, what would be concentration as %NaCl by mass in this solution?

First, write the formula for % mass which in this case would be the % NaCl since NaCl would be the solute and solutions are named after the solute.

Next, make a table of all the data needed for the formula.

% NaCl (w/w) = ?

mass water = 180 g (remember that the density of water = 1.0 g/ml)

mass NaCl = 20 g

Next, plug the numbers into the formula.

When dealing with two liquids, the concentration can also be expressed as the percent by volume.

Example: 200 ml of antifreeze is mixed with 0.8 liters of water, what would be % concentration of antifreeze by volume in this solution?

First, write the formula for % volume which in this case would be the % antifreezel since the antifreeze would be the solute and solutions are named after the solute.

Next, make a table of all the data needed for the formula.

% antifreeze (v/v) = ?

volume water = 800 ml (remember 0.8 liters = 800 ml)

volume antifreeze = 200 ml

Next, plug the numbers into the formula.

Sometimes, the concentration is expressed as a combination of the mass of the solute and the volume of the solution.

Example: 150 ml of ethanol is mixed with 1.5 liters of water, what would be % concentration of ethanol by mass/volume in this solution?

First, write the formula for % mass/volume which in this case would be the % ethanol since the ethanol would be the solute and solutions are named after the solute.

Next, make a table of all the data needed for the formula.

% NaOH (m/v) = ?

volume water = 1500 ml (remember 1.5 liters = 1500 ml)

mass ethanol = 118.5 g (density = 0.79 g/ml and mass = density x vol)

Next, plug the numbers into the formula.

Dilution Formula

The dilution formula is used to dilute a concentrated stock solution (these stock solutions are concentrated so as to save room during storage and costs in shipping) to a desired concentration.

C₁ = the initial concentration
C₂ = the final concentration
V₁ = the initial volume
V₂ = the final volume

Example: A stock solution of hydrochloric acid is has a concentration of 12M (C₁). 100 ml (V₂) of a 6M HCl (C₂) solution needs to be made from the stock solution. How many ml (V₁) of the stock solution needs to be diluted to 100 ml (V₂) to make the 6M (C₂) solution?

First using the dilution formula, solve for V₁ which is the volume of the 12M stock solution needed to make the 6M solution.

Next, make a table of the variables in the formula.

C₁ = 12M
C₂ = 6M
V₁ = ?
V₂ = 100 ml

Next, substitute the actual values for the variables in the formula.

The 50 ml indicates that you would take 50 ml of the 12M HCl solution and dilute it to 100 ml with water to make 100 ml of a 6M HCl solution.