January 27th - Lab: Testing the Stoichiometric Method

Today, we did a lab. The problem: does stoichiometry accurately predict the mass of products produced in chemical reactions?

In this experiment, we disolved [2.00g] of Strontium nitrate in 50 mL of water and then reacted it with [3.00g] of Copper (II) sulphate. The product is a precipitate (Strontium sulphate) and Copper (II) nitrate. After mixing the solutions, the precipitate is separated by filtration, dried, and weighed.

Behold Copper (II) sulphate!

The balanced equation for the reaction is:

Sr(NO₃)₂   +   CuSO₄   à   SrSO₄   +   Cu(NO₃)₂  

If 2.00g of Strontium nitrate completely reacts, 1.74g of Strontium sulphate is produced. Here are the calculations:

2.00g   x   1 mol/(87.6g +  2(14.0g) +6(16.0g))   x   1/1   x   (87.6g + 32.0g + 4(16.0g))   =   1.74g

Carrying out the experiment, we first measured 3.00g of Copper (II) sulphate. We then dissolved it in 30mL of water. We did the same with 2.00g of Strontium nitrate. We poured the solutions together (remembering to pour along the stir rod) into a filter and funnel which emptied into an Erlenmeyer flask. It is important to note that we measured the mass of the filter paper before the filtration. After filtration, we poured the filtrate into the sink and placed the filter paper in the drying oven. We then measured the mass when it had dried. Subtracting the weight of the filter paper from the mass of both the precipitate and the filter paper, we found the mass of Strontium (II) sulphate.

Here is a collection of all of our observations:

• Mass of Copper (II) sulphate used: 3.00g
• Mass of Strontium nitrate used: 2.00g
• Mass of filter paper: 2.27g
• Mass of precipitate and filter paper: 4.13g
• Mass of precipitate: 1.86g

The number of moles of precipitate formed was .0101 mol. Here are the calculations:

1.86g  x   mol/183.7g   =   0.0101 mol

Strontium nitrate being formed.

According to our original calculations above, we should have had [g] of precipitate. Therefore, we have a slight margin of error. The percent error is:

100((1.86g - 1.74g)/1.86g)   =   6.45%

Some reasons for our percent error are:

1. The beaker was contaminated. The previous class left Strontium nitrate inside the beaker, which added to our mass of 2.00g. 
2. The filter may not have completely dried. The water that remained in the filter could have contributed to the extra mass.
3. Some estimation was used. Human error is also possible. 

Posted by Michael.