K and Br can form an ionic bond. Since potassium is a metal (has a small electronegativity) and bromine is a nonmetal (has a large electronegativity), potassium can give an electron to bromine creating potassium bromide.
I hope this helps. Let me know if anything is unclear.
The problem ask on which of the following reactions and subsequent values and the value is given on you question. Base on your data and also to my formulation, solution and further simplification about the problem and the given values, i came up with an answer of <span>∆Hrxn = 5∆Hf(O2(g)) + 2∆Hf(C2H2(g)) - 2∆Hf(H2O(g)) - 4∆Hf(CO2(g)).</span>
Because it doesn't effect composition
3. a) 320 mL x 1 L/1000 mL = 0.320 L
b) 948 mL x 1 L/1000 mL = 0.948 L
c) 1834 mL x 1 L/1000 mL = 1.834 L
d) 3267 mL x 1 L/1000 mL = 3.267 L
4. a) 0.75 L x 1000 mL/1 L = 750 mL
b) 0.28 L x 1000 mL/1 L = 280 mL
c) 1.32 L x 1000 mL/1 L = 1320 mL
d) 2.4 L x 1000 mL/1 L = 2400 mL