Acetaldehyde (CH3CHO) is an important chemical both industrially and biologicall
ID: 525270 • Letter: A
Question
Acetaldehyde (CH3CHO) is an important chemical both industrially and biologically. For instance, it is a (somewhat toxic) intermediate in the body's metabolism of ethanol into acetic acid, and thus is possibly implicated in the "hungover" symptoms of someone who has had too much to drink the night before. In aqueous solution, it establishes an equilibrium with a hydrated form, shown below. CH3CHO (aq) + H2O (l) <-- --> CH3CH(OH)2 (aq) You start with an aqueous sample, already at equilibrium, with the concentration of CH3CH(OH)2 (the hydrated form) at a concentration of 2.60 M. You have no information about whether there is any of the anhydrous form (CH3CHO) initially in the flask, and if so, how much. If you add 2.0 M of CH3CHO to the reaction flask, and as the equilibrium is being restored the amount of CH3CH(OH)2 changes by 1.13 M, what is the final amount of CH3CHO?
Explanation / Answer
CH3CHO (aq) + H2O (l) <-- --> CH3CH(OH)2 (aq)
initially
in equilibrium
CH3CH(OH)2 = 2.6
CH3CHO = x
K = 2.6 / x
after adding 2 M of
CH3CH(OH)2 = 2.6
CH3CHO = x + 2
in equilibrium
CH3CH(OH)2 = 2.6 - y
CH3CHO = x + 2 - y
and we know that
CH3CH(OH)2 = 2.6 - y = 1.13
so
y = 2.6-1.13 = 1.47
now
CH3CHO = x + 2 - 1.47 = x + 0.53
substitute in K = 2.6 / x
2.6 / x = 1.13 / ( x + 0.53)
solve for x
2.6/1.3 = x/ ( x + 0.53)
2(x+0.53) = x
2x + 2*0.53 = x
x = 1.06 M
CH3CHO = x + 2 - 1.47 = x + 0.53 1.06 + 0.53 = 1.59 M