A Thomas flowmeter is a device in which heat is transferred at a measured rate from an electric coil to a flowing fluid, and the flow rate of the stream is calculated from the measured increase of the fluid temperature. Suppose a device of this sort is inserted in a stream of nitrogen, the current through the heating coil is adjusted until the wattmeter reads 1.25 kW, and the stream temperature goes from 30°C and 110 kPa before the heater to 34°C and 110 kPa after the heater.
(a) If the specific enthalpy of nitrogen is given by the formula H^ ( kJ / kg ) = 1.04 [ T ( ° C ) − 25 ] Hˆ(kJ/kg) =1.04[T(°C)−25], what is the volumetric flow rate of the gas (L/s) upstream of the heater (i.e., at 30°C and 110 kPa)?

The balanced reaction for the above reaction is;
Cu + 2AgNO₃ ---> Cu(NO₃)₂  +  2Ag
we need to first find the limiting reactant 
stoichiometry of Copper to AgNO₃ is 1:2
the number go Copper moles are 4.2 mol
the number of AgNO₃ moles are 6.3 mol
if AgNO₃ is the limiting reactant 
if 2 mol of AgNO₃ reacts with 1 mol of Cu 
then 6.3 mol of AgNO₃ reacts with - 1/2 x 6.3 mol = 3.15 mol of Cu
and 4.2 mol of Cu is present therefore AgNO₃ is the limiting reactant and Cu is in excess. 
stoichiometry of AgNO₃ to Ag is 2:2
therefore number of AgNO₃ moles reacted = number of Ag moles formed
number of Ag moles formed = 6.3 mol

excess reactant is copper 
only 3.15 mol were used up but 4.2 mol were present initially
the amount of excess reactant moles used = 4.2 - 3.15 = 1.05 mol of copper is in excess