Chemistry .. Please Help me?

Question

Consider the balanced equation:

2A + 5B ↔ 3C + D

8 mol A are mixed with 19 mol B at an elevated temperature. After 4 mol of A have reacted, how much B is left?

Please show me the steps I have many more similar problems to do like this.. it will be greatly appreciated.. Thanks alot

Answer 1

2A+5B<--->3C+D

You start with 8 moles of A which is 4 times the coefficient
19 moles of B is more than needed
4 moles of A have reacted, that is 2 times the coefficient
2 times 5 is 10 moles of B needed
9 moles of B are left

Answer 2

This looks like a nice question that illustration the concepts behind a limiting reagant and sticks with some not too hard ratios.
Your teacher or the text book probably showed you a way to do this with algebra. I STRONGLY suggest you master that IF you want to move on in chemistry. I you just want to get over this hump, take another aproach.

You can read the equation into words as: TWO moles of compound A will react with FIVE moles of compound B to yield THREE moles of compound C and ONE mole of compound D. You REALLY have to understand this! If you don't already have a sense for what this mean it would be SMART try and model something. I have even had students do it with symbols:
# (represent A)
$ (represent B)
; (represent C)
% (represent D)

SO
## + $$$$$ -> ;;; + %

NOW, they present you with

##
##
and
$$$$$
$$$$$
$$$$$
$$$$
(I deliberately lined it up to show the same unit at the original equation)
AND then
the problem asks : IF 4A has reacted, how much B is left?

To solve:
for every "line" of A that reacts, cross out a "line" of B. Count the remaing #.

No math!

Answer 3

From the equation, each TWO moles of A require FIVE moles of B. Therefore, FOUR (2X2) moles of A require (2X5) moles of B=10 moles.
19-10=9 moles left!

Answer 4

Ask A Chemisty Teacher.