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The lab procedure that my class used can be found in the photo attachment.

The purpose of the lab was to determine the equation for the decomposition of sodium bicarbonate. The two possible equations were:

  1. $\ce{2NaHCO -> Na2CO3 + CO2 + H2O}$
  2. $\ce{2NaHCO -> Na2O + 2CO2 + H2O}$

I understand that the correct equation is supposed to be the first one where sodium carbonate was produced. However, through calculations using the ratio between reactant and solid product, my experiment showed that equation 2 where sodium oxide was produced is supposed to be correct.

I know that this is caused by the mass of product I produced which was 0.716 grams rather than the expected expected 1.187 grams. However, what I do not understand is what errors could have potentially lead to this less than expected mass?

enter image description here

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  • $\begingroup$ You calculated your expected weight wrong. Edit your question to show that calculation. $\endgroup$ – MaxW Feb 5 '17 at 17:21
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    $\begingroup$ Your equations contain an incorrect formula for sodium bicarbonate. If you used that in your calculations, it would give the wrong expected weight if product. $\endgroup$ – iad22agp Feb 6 '17 at 14:04
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I understand that the correct equation is supposed to be the first one where sodium carbonate was produced. However, through calculations using the ratio between reactant and solid product, my experiment showed that equation 2 where sodium oxide was produced is supposed to be correct.

So now you're faced with a decision. You can either:

  • Massage the result of your experiment until it comes up with the answer you think is "supposed" to be correct.
  • Report the result of your experiment as it actually happened.

If you take the first route, I'm sure we can come up with some convincing explanations. Maybe your starting sodium bicarbonate contained water (so it wasn't 100% pure) and that water was driven off in the heating. Maybe you spilled some as you put it in the crucible. Maybe some of the mixture in the crucible splashed out during the reaction. Maybe you read the scale wrong. Maybe there is another reaction that also happens, perhaps involving the air, or the crucible, or the gas from the bunsen burner.

But I think it would be much more honest to report your result as it actually happened. Your data supports the hypothesis that the sodium bicarbonate decomposed to sodium oxide. In fact, the agreement with that hypothesis is quite strong, and the agreement with the supposedly correct hypothesis is terrible. It could be that this is due to an error you made... or maybe the stupid sodium bicarbonate didn't get the memo that, according to the teacher (or the text or Wikipedia or whatever), it was supposed to decompose to sodium carbonate, so it just went ahead and decomposed all the way to sodium oxide.

I realize you're probably worried about being marked down on your lab report because you came to the "wrong" conclusion. Any decent teacher, though, would give you much more credit for correctly interpreting your data and coming to a conclusion based on the data, even if it's wrong, than for massaging your data to support a preconceived hypothesis.

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  • $\begingroup$ maybe the OP could also study the calculations to find the error. $\endgroup$ – MaxW Feb 5 '17 at 18:57
  • $\begingroup$ We should use this answer as a go-to in case we encounter others asking how to explain unexpected results by fabrication. $\endgroup$ – Jan Nov 7 '17 at 9:14
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The reactions would be better written as:

  1. $\ce{2NaHCO3 ->[\Delta] Na2CO3 + CO2 ^ + H2O ^}$
  2. $\ce{2NaHCO3 ->[\Delta] Na2O + 2CO2 ^ + H2O ^}$

$\ce{NaHCO3}$, MW = 84.01
$\ce{Na2CO3}$, MW = 105.99
$\ce{Na2O}$, MW = 61.98

  1. $\ce{2NaHCO3 ->[\Delta] Na2CO3 + CO2 ^ + H2O ^}$

Starting with 2.0 grams $\ce{NaHCO3}$, the expected amount of $\ce{Na2CO3}$ is:

$g_{\ce{Na2CO3}} = \dfrac{1}{2}\cdot\dfrac{2.0}{84.01}\cdot105.99 = 1.26$

  1. $\ce{2NaHCO3 ->[\Delta] Na2O + 2CO2 ^ + H2O ^}$

Starting with 2.0 grams $\ce{NaHCO3}$, the expected amount of $\ce{Na2O}$ is:

$g_{\ce{Na2CO3}} = \dfrac{1}{2}\cdot\dfrac{2.0}{84.01}\cdot61.98 = 0.74$

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What you may have done was subtracted your final resulting weight of your crucible and Sodium Hydrogen Carbonate from the original weight of your Sodium Hydrogen Carbonate and Crucible before heating. This would have yielded an answer similar to the one you got. However, the correct procedure would be to subtract the weight of the original crucible from your final resulting weight of your Sodium Hydrogen Carbonate and Crucible. This would give you just the weight of how much Sodium Hydrogen Carbonate you have left, instead of your original mistake which only gives you the difference between your starting amount of Sodium Hydrogen Carbonate and your final amount.

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