The method of continuous variations is a means of determining the stoichiometric mole ratio of the reactants in a chemical reaction. The stoichiometric ratio, as given by the coefficients in the balanced chemical equation, represents the ratio at which chemicals must be combined to produce all product with no excess reactant. Since there is no “wasted” reactant, the maximum amount of product is made for the given amount of both reactants.

In the method of continuous variations, several trials are performed. In each trial, there is a fixed amount of total reactant (measured by moles or volume or mass, etc.), but with the two reactants mixed together in different ratios. Some measure of the amount of product is made for each trial (volume of gas produced, mass of precipitate, color of product solution, etc.). When the amount of product is maximized, the stoichiometric ratio has been used. A similar technique fixes the amount of one reactant and increases the other until the maximum amount of product is reached.

In this lab the two chemicals being mixed are solutions of known molarity. The total volume of solutions is kept constant over several trials. The chemicals will release heat when mixed together and raise the temperature of the mixture. Since the volume of the solutions mixed is held constant, the temperature change recorded will be proportional to the amount of heat produced and therefore also the “amount of reaction” that occurred.

Objectives:

- To graphically determine the stoichiometric mole ratio of a reaction.

CAUTIONS:

- All solutions are basic and can be harmful to skin and eyes. Sodium hypochlorite can also bleach clothing. Wear goggles, gloves and aprons.

Pre-lab: complete on a separate sheet.

1. Accurately graph the following data gathered from an experiment which mixes 0.5 M solutions of AgNO3 and K2CrO4. The x-axis should plot the volume of AgNO3 reading from left to right (or the volume of K2CrO4 reading from right to left.) Draw two best fit lines, one for the “up” slope and one for the “down” slope... see example to the right of the data. (Also consult your graphing skills handout if you have not yet drawn a graph)

|trial |mL AgNO3 |mL K2CrO4 |mass of precipitate (g) | |1 |5 |45 |1.7 | |2 |15 |35 |5.0 | |3 |25 |25 |8.3 | |4 |30 |20 |10.0 | |5 |35 |15 |9.9 | |6 |40 |10 |6.6 | |7 |45 |5 |3.3 |

a) Use the intersection of best fit lines to find the theoretical maximum amount of product. b) Use the x-values to find the stoichiometric volume ratio

c) Use the answer from b) and given molarities to find the stoichiometric mole ratio. d) How many values were used to plot your “up” line? How many values were used to plot your “down” line? Are these enough values to plot a good line?

2. What single temperature change should be recorded if 10 mL of solution A at 20°C is mixed with 10 mL of solution B at 24.0°C and the final temperature reaches 28.0°C?

3. What single temperature change should be recorded if 10 mL of solution A at 20°C is mixed with 30 mL of solution B at 24.0°C and the final temperature reaches 28.0°C?

4. List four dependent variables that are often used for the method of continuous variations.

5. (Skip this...