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decomposition of potassium hydrogencarbonate

Submitted by abc123 on Tue, 05/08/2012 - 08:03
2KHCO3      ------>  K 2CO3   +   CO     +    H2 O
 
           
A student was asked to check the purity of a sample of potassium hydrogencarbonate.
The student weighed a clean, dry crucible, and transferred 1.00g of the potassium hydrogencarbonate to the crucible. A lid was placed on the crucible and the crucible was then heated for a few minutes.  After cooling, the mass of the crucible and its contents was recorded.
 
         
Give one reason why the use of a lid improves the accuracy of the experiment. 
 
State one reason why the use of a very small amount of potassium hydrogencarbonate could lead to a less accurate result. 
 

Equation should show CO2 and not CO as a product.

A lid prevents loss of solid material during heating but allows the gas to escape as long as it is not tight fitting. So you only want to lose volatile materials and not solids by sublimation

 If the balance weighs to 2 dec places it is accurate to +/- 0.005g. The percentage error if 2.00g is used is (+/- 0.005/2.000) x100 = +/-0.25% or if 10.000g is used, then the error is (+/- 0.005/10.000) x100 = +/-0.05%. S a lower percentage error if a larger mass is used. Really you should also add the crucible to this as it is included in the weighing eg if the crucible weighs 30.00g, The percentage error if 2.00g is used is (+/- 0.005/32.000) x100 = +/-0.015625% or if 10.000g is used, then the error is (+/- 0.005/40.000) x100 = +/-0.0125%. As you can see, the weighing errors are lower the greater the masses weighed