Problem: Calculate the concentration of each species present in a 0.010-M solution of phthalic acid, C6H4(CO2H)2.C6H4(CO2 H)2(aq) + H2 O(l) ⇌ H3 O+(aq) + C6 H4 (CO2 H) (CO2 )−(aq)        Ka = 1.1 × 10−3C6H4(CO2 H)(CO2)(aq) + H2 O(l) ⇌ H3O+(aq) + C6 H4(CO2)2 2−(aq)            Ka = 3.9 × 10−6

FREE Expert Solution

We are being asked to calculate each species present in a 0.010-M solution of phthalic acid, C6H4(CO2H)2.

Step 1: Construct an ICE chart for the equilibrium reaction.

Since we’re dealing with a weak acid and Ka is an equilibrium expression, we will have to create an ICE chart to determine the equilibrium concentration of each species

We will use the first equilibrium reaction given:

       C6 H4(CO2 H)2(aq) + H2 O(l) ⇌ H3 O+(aq) + C6 H(CO2 H) (CO)(aq)

        phthalic acid (aq) + H2 O(l) ⇌ H3 O+(aq) + biphthalate (aq)



Step 2: Write the Ka is an equilibrium expression.

Ka=productsreactantsKa=[biphthalate][H3O+][phthalic acid]

Solids and liquids are not included in the expression


Step 3: Calculate for the equilibrium concentration.

1.1×10-3=xx0.01-x1.1×10-3=x2[0.01-x]

phthalic acidiKa=0.01 M1.1×10-3500;     we cannot ignore x in the denominator

(1.1x10-3)=x20.015-x(1.1x10-3)0.01-x=x2x2+0.0011x-0.000011=0


Now, we will use the quadratic formula to solve for x. The quadratic formula is:

x=-b±b2-4ac2a

From the equation, we have a = 1, b = 0.011, and c = -0.000011


Solve for x using the solver function from the calculator:


x = -0.0039  (can't be negative) ; x =0.002812


Hence x = 0.002812 M = [bipthalate] = [C6 H(CO2 H) (CO)] = [H3O+]

phtalic acid = 0.010-0.002812 = 0.007188 M


Next is the 2nd equilibrium reaction given:

       C6H4(CO2 H)(CO2)(aq) + H2 O(l) ⇌ H3O+(aq) + C6 H4(CO2)2−(aq)            Ka = 3.9 × 10−6

         biphthalate (aq) + H2 O(l) ⇌ H3 O+(aq) + phthalate (aq)


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Problem Details

Calculate the concentration of each species present in a 0.010-M solution of phthalic acid, C6H4(CO2H)2.

C6H4(CO2 H)2(aq) + H2 O(l) ⇌ H3 O+(aq) + C6 H(CO2 H) (CO)(aq)        Ka = 1.1 × 10−3
C6H4(CO2 H)(CO2)(aq) + H2 O(l) ⇌ H3O+(aq) + C6 H4(CO2)2−(aq)            Ka = 3.9 × 10−6

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What scientific concept do you need to know in order to solve this problem?

Our tutors have indicated that to solve this problem you will need to apply the Diprotic Acid concept. You can view video lessons to learn Diprotic Acid. Or if you need more Diprotic Acid practice, you can also practice Diprotic Acid practice problems.

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