Why percent yield is sometimes less than 100% in chemistry experiments?

[harveycannon]

Homework Statement

I'm learning about percent yield in chemistry labs and noticed that the percent yield is often less than 100%.

What are the most common reasons for this?

Relevant Equations

Percent Yield = (Actual Yield ÷ Theoretical Yield) × 100

Also, when calculating percent yield, do we always use the same formula?

I understand that percent yield compares actual and theoretical yield, but I want to make sure I'm doing the calculation correctly.

 

[Mayhem]

Small thing. Percent yield = (actual yield / theoretical yield) * 100.

Without the 100 factor, you just get a ratio, not a % expression.

 

[harveycannon]

Sorry, I forgot to multiply 100 in the equation. It's corrected now.

 

[harveycannon]

For my case:

• Theoretical yield = 10.3 g
• Actual yield = 11.25 g

After calculating the percent yield = 109.223%

I also verified my calculation here: https://theonlinetools.net/percent-yield-calculator.html

My question is:
Does this mean the reaction produced more product than expected, or is it usually caused by measurement errors or impurities?

I'd appreciate an explanation of why this happens in chemistry experiments.

 

[Borek]

Does this mean the reaction produced more product than expected, or is it usually caused by measurement errors or impurities?

No way to produce more, mass conservation doesn't allow for that (in a way stoichiometry is an elaborate way of saying "matter is not lost nor created"). So it is always a problem with measuring or impurities (if it was a water based reaction most common problem is drying the product before weighing).

 

[Mayhem]

I see what you are asking. Understand that the problem you stated in the thread title is opposite of what actually meant to ask.

When the measured yield is higher than the theoretical yield, it is either because the theoretical yield was incorrectly calculated, or because the isolated product contains impurities (this is most likely).

 

[Hornbein]

I see what you are asking. Understand that the problem you stated in the thread title is opposite of what actually meant to ask.

When the measured yield is higher than the theoretical yield, it is either because the theoretical yield was incorrectly calculated, or because the isolated product contains impurities (this is most likely).

Another source could be errors in measurement. More reactants than measured or misweighed the product.

When I say percent yield I mean weight of the desired product divided by the weight of the reactants times 100%. This should always be less than or equal to 100% and can be quite low. But in this case the "theoretical yield" might be the mass of product expected to be obtained by skilled chemists. I can't tell which definition they are using.

 

[TensorCalculus]

For my case:

• Theoretical yield = 10.3 g
• Actual yield = 11.25 g

After calculating the percent yield = 109.223%

I also verified my calculation here: https://theonlinetools.net/percent-yield-calculator.html

My question is:
Does this mean the reaction produced more product than expected, or is it usually caused by measurement errors or impurities?

I'd appreciate an explanation of why this happens in chemistry experiments.

Depending on the reaction, it might be that side reactions of one/some of the reactants with the the air have also skewed your result, whether that's from reactions with the air causing the products to be impure or the air contains one of the reactants causing more of the product to be made than expected (though the latter is unlikely given the fact that even if one of your reactants is something abundant in the air like nitrogen or oxygen, you've probably set up your reaction such that that's in excess so more of that won't produce more product anyways)