2. A student thought he carefully followed the directions in the experimental pr

Question

2. A student thought he carefully followed the directions in the experimental procedure, and he expected 2.63g of product (Aspirin). However, he isolated 3.05g of product. what would account for the excess mass? 2. A student thought he carefully followed the directions in the experimental procedure, and he expected 2.63 g of product. However, he isolated 3.05 g of product. What would account for the excess mass? 3. The above student also found that his product tested positive with 1% iron(III) chloride solution. What does this test indicate? 4. Another student also expected 2.63 g of product, but isolated only 2.45 g. What is the percentage yield? Show your work. 375

Explanation / Answer

The reaction to generate aspirin, is the following:

Salicic Acid + Acetate anhydride -------------> Aspirin.

the proportion of moles is 1:1

So, first let's write the molecular weight of salicic acid, acetate anhydride and aspirin (labeled as 1, 2 and 3)

MM 1 = 138.12 g/mol ; MM 2 = 102.09 g/mol ; MM 3 = 180.16 g/mol

As I stated above, the number of moles of each reactants has a proportion of 1:1, so, in order to get the mass of the expected product, the number of moles of the reactants should be the same.

This means that for the expected product, the number of moles:

moles 3 = 2.63 g / 180.16 = 0.0146 moles

With this we can know the mass of salicid acid used:

mass 1 = 0.0146 * 138.12 = 2.0163 g

So, the studen should have weight 2.0163 g. If the product was 3.05 g (excess of 15%), then he probably weight a little more of salicid acid:

moles 3 = 3.05 / 180.16 = 0.0169 moles

mass 1 = 0.0169 * 138.12 = 2.3342 g

So, he probably weight this mass of salicic acid. And this probably happens because the balance was not calibrated correctly, so he through the weighted the correct mass and therefore, the product was not the expected. It can also happen that he weight (or measured) the incorrect amount of mass (or volume) or acetic anhydride.

3. That's a test for determining the presence of phenols or enols in a unknown sample. In this case, the aspirin or acetisalicic acid, has in it's structure an enol and therefore, the solution it's positive with this reactive.

4. If the product was 2.45, then the % yield is:

% = (2.45 / 2.63) * 100 = 93.16 %

Hope this helps.

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