Question

Which substance has a coefficient of 2 when you balance the equation for this redox reaction? cu(s) + hno3(aq) → cu(no3)2(aq) + no(g) + h2o(l) hno3(aq) no(g) h2o(l) cu(s)?

Answer 1

In this redox reaction, the Cu goes from oxidation state of (0) to (+2), therefore it oxidises. N in HNO₃ goes from oxidation state of (+5) to N in NO with oxidation state of (+2) and becomes reduced. 
Cu acts as the reducing reagent and HNO₃ is the oxidising agent.
oxidation half reaction
Cu ---> Cu²⁺ + 2e  --1)
reduction half reaction 
4H⁺ + 3e + NO₃⁻ ---> NO + 2H₂O --2)
to balance the number of electrons , 1) x3 and 2) x2
3Cu ---> 3Cu²⁺ + 6e
8H⁺ + 6e + 2NO₃⁻ ---> 2NO + 4H₂O 
add the 2 equations 
3Cu + 8H⁺ + 2NO₃⁻ --->  3Cu²⁺ + 2NO + 4H₂O 
add 6 nitrate ions to both sides to add up to 8 and form acid with 8H⁺ ions
3Cu + 8HNO₃ --->  3Cu(NO₃)₂ + 2NO + 4H₂O 
Balanced equation for the redox reaction is as follows;
3Cu(s) + 8HNO₃(aq) → 3Cu(NO₃)₂(aq) + 2NO(g) + 4H₂O(l)

NO has a coefficient of 2