Answer:- 38.2 g.
Solution:- The equation used for solving this type of calorimetry problems is:

where, q is the heat energy, m is mass, c is specific heat and delta T is the change in temperature.
Water temperature is increasing from 14.5 degree C to 50.0 degree C.
= 35.5 degree C
q is given as 5680 J and specific heat value is
.
The equation could be rearranged for m as:

Let's plug in the values in it:

m = 38.2 g
So, the mass of water in the kettle is 38.2 g.
When acids react with water, H ions are released which then combine with water molecules to form H₃O⁺
The molecular formula of calcium oxide - CaO
The molar mass of CaO - 40 + 16 = 56 g/mol
Which means that 1 mol weighs 56 g
Therefore 56 g of CaO is - 1 mol
Then 89.23 g is equivalent to - 1/56 x 89.23 = 1.6 mol of CaO
Reactant C is the limiting reactant in this scenario.
Explanation:
The reactant in the balanced chemical reaction which gives the smaller amount or moles of product is the limiting reagent.
Balanced chemical reaction is:
A + 2B + 3C → 2D + E
number of moles
A = 0.50 mole
B = 0.60 moles
C = 0.90 moles
Taking A as the reactant
1 mole of A reacted to form 2 moles of D
0.50 moles of A will produce
= 
thus 0.50 moles of A will produce 1 mole of D
Taking B as the reactant
2 moles of B reacted to form 2 moles of D
0.60 moles of B reacted to form x moles of D
= 
x = 2 moles of D is produced.
Taking C as the reactant:
3 moles of C reacted to form 2 moles of D
O.9 moles of C reacted to form x moles of D
= 
= 0.60 moles of D is formed.
Thus C is the limiting reagent in the given reaction as it produces smallest mass of product.