[roneddy]

In our experiment we used 200 grams of 6% by mass sodium hypochlorite bleach and added 10g of sodium sulphite to it. We were then asked to calculate the heat gained or lost by the surroundings. Knowing that it was exothermic since final temp was 56 degrees Celsius and initial was 23 degrees Celsius. This is what I have done:

Heat gained by surroundings = MCdeltaT
                                          = (200)(4.18)(56-23)

is the mass used correct? since I believe we assume the solutions are diluted.

[Borek]

No, total mass is not 200 g. You can't end with 200 g if you combine 200 g and 10 g.

There are several other things that I don't like about the question, but it is not clear to me how the original problem was worded, so I don't know if you just posted it this way, or was the original question unclear.