Question

A person’s blood alcohol (C2H5OH) level can be determined by titrating a sample of blood plasma
with a potassium dichromate solution. The balanced equation is

16H+ (aq) + 2Cr2O72-­‐(aq) + C2H5OH   → 4Cr3+(aq) + 2CO2(g) + 11H2O(l)

(a) Identify the reducing agent and oxidizing agent.

(b) If 45.32 mL of 0.04261 M dichromate ion is required to titrate 25.00 g of the

blood plasma, what is the mass percent of alcohol in the blood plasma

Answer 1

The balanced equation is

16H+ (aq) + 2Cr2O72-­‐(aq) + C2H5OH   → 4Cr3+(aq) + 2CO2(g) + 11H2O(l)

Oxidation number of Cr in Cr2O72- is +6

2x +7*-2= -2

X= +6

And Cr3+ is 3

Cr is reduced thus it is Oxidizing agent

H+ is reducing agent

(b) given that

45.32 mL of 0.04261 M dichromate ion is required to titrate 25.00 g of the blood plasma

Number of moles = molarity * volume in L

dichromate ion= 0.04261 *45.32/1000

= 0.00193 MOLES

Now calculate the mole of alcohol as follows:

0.00193 MOLES *1 mole alcohol /2 mole dichromate

= 0.000965 mole C2H5OH

Amount in g = molar mass* number of moles

= 46.07 g/mol *0.000965 mole C2H5OH

= 0.04446 g

mass percent of alcohol in the blood plasma = mass of alcohol/ sample mass*100

= 0.04446 g/25.00 g*100

= 0.18 %

Oxidation-reduction reaction or redox reaction:

A reaction in which one reactant is oxidized and another is reduced.

Oxidation:

Oxidation is a process in which either 1 or all following changes occurs:

1. Gaining of oxygen atoms
2. Loss of electrons
3. Loss of hydrogen atom.
4. Increasing oxidation number.

Reduction:

Reduction is a process in which either 1 or all following changes occurs:

1. Loss of oxygen atoms
2. Gaining of electrons
3. Gaining of hydrogen atom.
4. Decreasing oxidation number.

Oxidation and reduction process are reverse process.

Oxidizing agent:

Oxidizing agent is a reactant of a reaction which causes an oxidation through accepting electron.

Reducing agent:

Reducing agent is a reactant of a reaction which causes a reduction through donating electron.