Can anyone explain to me how Zinc is in EXCESS in this question?
A mass of 6.5 g of granulated zinc was reacted with 0.24 moles of nitric acid. The equation for the reaction is,
3Zn + 8HNO3 = 3Zn(NO3)2 + 2NO + 4H2O
6.5 g of zinc is 0.1 mole.
Ratio of equipment stars that Zn reacts with HNO3 in the ratio 3 : 8..
0.1 moles = 6.5 g Zn
0.24 moles represents the 8 so divide 0.24 by 8 to get the number of moles that would be represented by the number 1 in the ratio.
Zn : HNO3
3 : 8
0.09 : 0.24
Therefore only 0.09 moles of Zn is going to react with 0.24 moles HNO3:
0.1 moles that's given is too much or in excess.
So basically in a balanced equation like the one you gave, it is often assumed that all of the reacting chemicals change into products.
In that case, at the end of the reaction, no Zinc (Zn) or Nitric Acid (HNO3) will be left behind. However, if there is a shortage of, say, HNO3 then the reaction will stop when the HNO3 runs out. Some Zn will be left over, unable to react, as there is no more HNO3 to react with it. The Zn is said to be in excess.
So you want to prove that there is enough moles of Zn to fully react with the HNO3 without running out.
To do this you want to find out the no. moles of zinc , found by dividing the mass by the RAM (6.5/65.39). Which equals 0.0994 moles.
But the ratio is 3:8 ( as seen in the equation) so multiply the no. of moles of HNO3 by 3/8.
0.24(3/8) = 0.09
=> the HNO3 needs 0.09 moles to fully react and there is 0.094 moles of Zn present
=> The Zn is indeed in excess
Hope this helps and good luck with the colossal bitch that is chemistry