For the following reaction between Mohr\\\'s salt (iron as FeSO4(NH4)2SO4·6H2O)

Question

For the following reaction between Mohr's salt (iron as FeSO4(NH4)2SO4·6H2O) and potassium dichromate (dichromate as K2Cr2O7), determine the volume (in milliliters) of a 0.260 M solution of Mohr's salt that is needed to fully react with 0.0300 L of 0.260 M potassium dichromate. (The reaction is shown in its ionic form in the presence of a strong acid.)

For the same reaction, what volume (in milliliters) of 0.260 M potassium dichromate is required to fully react with 0.0300 L of a 0.260 M solution of Mohr's salt?

Explanation / Answer

6 FeSO4(NH4)2SO4 + K2Cr2O7 + 14 H{+}
12 NH4{+} + 12 SO4{2-} + 6 Fe{3+} + 2 K{+} + 2 Cr{3+} + 7 H2O

Or ionic equation:

Cr2O7 2- + 6Fe2+ + 15H+ --> 2Cr2+ + 6Fe3+ + 7H2O

Here 0.0300 L of 0.260 M potassium dichromate and .260 M solution of Mohr's salt

First calculate the number of moles of potassium dichromate:

Number of moles = molarity * volume in L

=0.260 Mole / L*0.0300 L

= 7.8*10^-3 moles K2Cr2O7

Now calculate the moles of Mohr's salt:

0.0078 moles K2Cr2O7 x (6Fe2+ / 1 Cr2O7 2-) = 0.0468 moles Fe2+

Molarity = number of moles / volume in L

OR

Volume in L = number of moles / molarity

= 0.0468 moles Fe2+ /0.260

= 0.180L

= 180 mL

For the same reaction, what volume (in milliliters) of 0.260 M potassium dichromate is required to fully react with 0.0300 L of a 0.260 M solution of Mohr's salt?

Cr2O7 2- + 6Fe2+ + 15H+ --> 2Cr2+ + 6Fe3+ + 7H2O

First calculate the number of moles of Mohr's salt:

Number of moles = molarity * volume in L

=0.260 Mole / L*0.0300 L

= 7.8*10^-3 moles Mohr's salt

Now calculate the moles of K2Cr2O7:

0.0078 moles Mohr's salt x ( 1 Cr2O7 2-/6Fe2+) = 0.0013 moles Cr2O7 2-

Molarity = number of moles / volume in L

OR

Volume in L = number of moles / molarity

= 0.0013 moles Cr2O7 2- /0.260

= 0.005L

= 5.0 mL

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