If I mix 300g of reactant A with 100g of reactant B in a close system how much will it weigh

well, since no mass is lost, ...

To find out the total weight of the reactants A and B when mixed together, you simply need to add the individual weights of the reactants.

Given:
Weight of reactant A = 300g
Weight of reactant B = 100g

To find the total weight, you add the two weights together:

Total weight = Weight of reactant A + Weight of reactant B

Total weight = 300g + 100g

Total weight = 400g

So, when you mix 300g of reactant A with 100g of reactant B in a closed system, the total weight will be 400g.