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C  Na₂ CO₃ + b  Cu SO₄ + a H2 O  ==> b  Na SO4 + b Cu O+ y CO₂  + a  H2 O

There is an intermediate product of  Cu (OH) CO3 to decompose in the products as shown.

basically in the aqua solution copper oxide and carbon dioxide are produced after the reaction..  We have to balance the above equation. 

The cork is to be put to prevent escape of any gases produced in the reaction. Here it is the CO2.  Otherwise there will be change in the mass measured after reaction.

C)    I am not sure I understood the problem correctly.

When the bucket is fully immersed, there is a buoyancy force acting on the bucket.  Hence the apparent weight of the bucket of water is equal to the actual weight in air - buoyancy force. 

Buoyancy force is =  weight of water displaced
         = volume of bucket immersed * density * g
Hence the bucket weighs less.

If the bucket is not immersed in water also it weighs a little less.

A bucket of water weighs less in a well probably because of the reduced gravitational attraction and reduced value of  g.

     g'  =  g ( 1 - h/Re),  where h = depth  of the well.  Re radius of Earth  and g is the acceleration due to gravity on the surface of Earth.

   g' is less than g.  Hence the weight ( mass * g) will be less when  the bucket is inside the well.

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