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Some Basic Concepts of Chemistry question

2021 · 25 Jul · Shift 1 · Q22
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Some Basic Concepts of Chemistry question

2021 · 25 Jul · Shift 1 · Q22

JEE MainChemistrySome Basic Concepts of ChemistryNumerical+4 / −1
When 10 mL of an aqueous solution of Fe2+Fe^{2+}Fe2+ ions was titrated in the presence of dil H2SO4H_2SO_4H2​SO4​ using diphenylamine indicator, 15 mL of 0.02 M solution of K2Cr2O7K_2Cr_2O_7K2​Cr2​O7​ was required to get the end point. The molarity of the solution containing Fe2+Fe^{2+}Fe2+ ions is x ×\times× 10 −-− 2 M. The value of x is ‾\underline{\hspace{2cm}}​. (Nearest integer)
Numerical answer
View written solutionFree

Correct answer: 18

  1. Write the redox reaction in acidic medium

In acidic medium, dichromate oxidizes Fe2+Fe^{2+}Fe2+ to Fe3+Fe^{3+}Fe3+:

Cr2O72−+14H++6Fe2+→2Cr3++7H2O+6Fe3+Cr_2O_7^{2-} + 14H^+ + 6Fe^{2+} \rightarrow 2Cr^{3+} + 7H_2O + 6Fe^{3+}Cr2​O72−​+14H++6Fe2+→2Cr3++7H2​O+6Fe3+

So, 1 mole of K2Cr2O7K_2Cr_2O_7K2​Cr2​O7​ reacts with 6 moles of Fe2+Fe^{2+}Fe2+.

  1. Calculate moles of K2Cr2O7K_2Cr_2O_7K2​Cr2​O7​ used

Given:

  • Volume of K2Cr2O7K_2Cr_2O_7K2​Cr2​O7​ solution =15 mL=0.015 L= 15\,\text{mL} = 0.015\,\text{L}=15mL=0.015L
  • Molarity =0.02 M= 0.02\,\text{M}=0.02M

Moles of K2Cr2O7K_2Cr_2O_7K2​Cr2​O7​:

n=M×V=0.02×0.015=3.0×10−4n = M \times V = 0.02 \times 0.015 = 3.0 \times 10^{-4}n=M×V=0.02×0.015=3.0×10−4

  1. Calculate moles of Fe2+Fe^{2+}Fe2+ present

Using the stoichiometric ratio 1:61:61:6,

moles of Fe2+=6×3.0×10−4=1.8×10−3\text{moles of } Fe^{2+} = 6 \times 3.0 \times 10^{-4} = 1.8 \times 10^{-3}moles of Fe2+=6×3.0×10−4=1.8×10−3

  1. Calculate molarity of Fe2+Fe^{2+}Fe2+ solution

Volume of Fe2+Fe^{2+}Fe2+ solution =10 mL=0.010 L= 10\,\text{mL} = 0.010\,\text{L}=10mL=0.010L

M=nV=1.8×10−30.010=0.18 MM = \frac{n}{V} = \frac{1.8 \times 10^{-3}}{0.010} = 0.18\,\text{M}M=Vn​=0.0101.8×10−3​=0.18M

Given that molarity is written as:

x×10−2 Mx \times 10^{-2}\,\text{M}x×10−2M

So,

0.18=18×10−20.18 = 18 \times 10^{-2}0.18=18×10−2

Hence,

x=18x = 18x=18

  1. Comparison with stored answer

Derived answer is 181818, which matches the stored correct answer.

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