April 16, 2024, 01:46:14 PM
Forum Rules: Read This Before Posting

### Topic: Working out concentration of reactant given concentration of product and time?  (Read 3263 times)

0 Members and 1 Guest are viewing this topic.

#### bmu123

• Regular Member
• Posts: 31
• Mole Snacks: +0/-0
##### Working out concentration of reactant given concentration of product and time?
« on: February 21, 2015, 09:51:54 AM »
I know I'm probably being thick here but for the reaction 2[A] -> [B ], how would I work out the value of [A] given the values of [B ] at given times?
So I'm given
Time/s                          0    10         20       30       ∞
[B ]/moldm-3  0     0.089    0.153   0.200   0.312

So I'm guessing at ∞ [A] = 0 but then I just can't think of what I need to do to work out the rest.
« Last Edit: February 21, 2015, 11:12:43 AM by sjb »

#### sjb

• Global Moderator
• Sr. Member
• Posts: 3652
• Mole Snacks: +222/-42
• Gender:
##### Re: Working out concentration of reactant given concentration of product and time?
« Reply #1 on: February 21, 2015, 11:13:37 AM »
I know I'm probably being thick here but for the reaction 2[A] -> [B ], how would I work out the value of [A] given the values of [B ] at given times?
So I'm given
Time/s                          0    10         20       30       ∞
[B ]/moldm-3  0     0.089    0.153   0.200   0.312

So I'm guessing at ∞ [A] = 0 but then I just can't think of what I need to do to work out the rest.

What does the reaction stoichoimetry tell you?

#### Plontaj

• Regular Member
• Posts: 29
• Mole Snacks: +4/-1
##### Re: Working out concentration of reactant given concentration of product and time?
« Reply #2 on: February 21, 2015, 12:13:21 PM »
If the reaction is irreversible, it is quite simple to determine the concentration of A. If it is reversible, will be a little harder.

#### bmu123

• Regular Member
• Posts: 31
• Mole Snacks: +0/-0
##### Re: Working out concentration of reactant given concentration of product and time?
« Reply #3 on: February 21, 2015, 12:20:59 PM »
It is irreversible. So if there are 2 moles of A for every mole of B so double it?

#### Borek

• Mr. pH
• Deity Member
• Posts: 27647
• Mole Snacks: +1800/-410
• Gender:
• I am known to be occasionally wrong.
##### Re: Working out concentration of reactant given concentration of product and time?
« Reply #4 on: February 21, 2015, 06:34:15 PM »
Can you use the data given to calculate [ B] at t=∞?
ChemBuddy chemical calculators - stoichiometry, pH, concentration, buffer preparation, titrations.info

#### bmu123

• Regular Member
• Posts: 31
• Mole Snacks: +0/-0
##### Re: Working out concentration of reactant given concentration of product and time?
« Reply #5 on: February 22, 2015, 07:55:13 AM »
So if at ∞ [ B] = 0.312, there must have been 0.624 moles of A to begin with?
So could I then say
for 30 seconds [ B] = 0.200; 0.312 - 0.200 = 0.112 and then x 2 so [A] is 0.224?

So
Time    0         10        20       30       ∞
[ B]      0          0.089   0.153   0.200  0.312
[A]      0.624    0.446   0.318   0.224  0
« Last Edit: February 22, 2015, 01:46:41 PM by Borek »

• Mr. pH