Using Method of Continuous Variations to Determine Mole Ratio of Reactants

Using the Method of Continuous Variations to Determine the Mole Ratio between Reactants
Abstract:
The purpose of this lab was to find the molar ratio of NaClO and an unknown substance labeled solution “B”. Using the method of continuous variation the ratio of the two solutions were changed but kept equal to 50 mL. The reaction was an exothermic oxidation-reduction and the temperature change was measured to determine which ratio of NaClO to solution “B” was optimal. The ratio with the highest temperature change was 40mL of NaClO to 10 mL of solution “B”. Once simplified, this means the ratio is 4:1. Now that the mole ratio was determined it can be used in stoichiometry conversions.
Introduction:
In order to determine the mole ratio of the two reactants, NaClO and Solution B, the method of continuous variations was used. Method of continuous variations involves changing the ratios of the two reactants to find the optimal ratio at which the temperature is found to be the highest. The change of temperature was measured since the heat is directly proportional to the amount of reaction and all the reactions are exothermic. Each measurement had a different mole ratio of reactants and the total volume of the solutions was kept constant for all of the trials. Stoichiometry is the mole ratio for the reactants in the balanced chemical equation and also forms the most product. The optimal ratio is the stoichiometric ratio and creates the most heat and has maximum temperature change since it consumes the greatest amount of reactants. All the reactions were oxidation-reduction reactions with sodium hypochlorite as the oxidizing agent. Once the maximum temperature of the final solutions with three measurements on each side of the greatest temperature difference was found, the data was plotted onto a graph and the intersection of the lines of best fit showed the stoichiometric mole ratio of the reactants.
Procedures:
Firstly, 175 mL of NaClO and the unknown solution “B” were measured, placed into separate beakers, and made sure they both had the same temperature. Then, in order to find a trend using the method of continuous variations, different ratios of NaClO and solution “B” that added up 50 mL were combined in a Styrofoam cup. The two substances were mixed with a thermometer to measure and calculate the change in temperature. The cup was then rinsed out and dried for another trial. The process of mixing different ratios of NaClO and the unknown solution was repeated until there were three data points on both sides of the highest temperature taken.
Data & Calculations

Data & Calculations (Continued): Change in Temperature
From 5mL NaClO: 45mL Solution B Ratio
Max. Temp. Solution Mixture 28.0 oC
- Initial Temp. - 24.1 oC

Change in Temperature 3.9 oC

Analysis & Conclusions: Using the method of continuous variations the mole ratio between NaClO and solution “B” was found to be 4:1. This conclusion was made because the highest recorded temperature, which was 48.5 °C, came from trial 6, which was a ratio of 40 mL of NaClO to 10 mL of solution “B”. This means 4:1 is the optimum ratio. There were no problems or errors during the lab that were noticed or that affected the data. Further tests would include more trials to see if the ratio determined was correct.
References:
Zumdahl, Steven S., and Susan A. Zumdahl. Chemistry. 7th ed. Boston: Houghton Mifflin, 2007. Print.

References: Zumdahl, Steven S., and Susan A. Zumdahl. Chemistry. 7th ed. Boston: Houghton Mifflin, 2007. Print.