Question

Enter your answer in the provided box. 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(aq) → 4Cr3 (aq) 2CO2(g) 11H2O(l) If 35.46 mL of 0.05961 M Cr2O72− is required to titrate 26.60 g of plasma, what is the mass percent of alcohol in the blood

Answer 1

Answer: The mass percent of alcohol in blood is 0.183 %

Explanation:

To calculate the moles for given molarity, we use the equation:

$\text{Molarity of the solution}=\frac{\text{Moles of solute}}{\text{Volume of solution (in L)}}$

Molarity of $Cr_2O_7^{2-}$ solution = 0.05961 M

Volume of solution = 35.46 mL = 0.03546 L   (Conversion factor:  1 L = 1000 mL)

Putting values in above equation, we get:

$0.05961M=\frac{\text{Moles of }Cr_2O_7^{2-}}{0.03546L}\\\\\text{Moles of }Cr_2O_7^{2-}=(0.05961mol/L\times 0.03546L)=2.11\times 10^{-3}mol$

The given chemical reaction follows:

$16H^+(aq.)+2Cr_2O_7^{2-}(aq.)+C_2H_5OH(aq.)\rightarrow 4Cr^{3+}(aq.)+2CO_2(g)+11H_2O(l)$

By Stoichiometry of the reaction:

2 moles of dichromate ions react with 1 mole of ethanol.

So, $2.11\times 10^{-3}mol$ of dichromate ions will react with = $\frac{1}{2}\times 2.11\times 10^{-3}=1.06\times 10^{-3}mol$

• To calculate the number of moles, we use the equation:

$\text{Number of moles}=\frac{\text{Given mass}}{\text{Molar mass}}$

Moles of ethanol = $1.06\times 10^{-3}mol$

Molar mass of ethanol = 46.07 g/mol

Putting values in above equation, we get:

$1.06\times 10^{-3}mol=\frac{\text{Mass of ethanol}}{46.07g/mol}\\\\\text{Mass of ethanol}=(1.06\times 10^{-3}mol\times 46.07g/mol)=0.0488g$

• To calculate the mass percent of ethanol in blood, we use the equation:

$\text{Mass percent of ethanol}=\frac{\text{Mass of ethanol}}{\text{Mass of blood plasma}}\times 100$

Mass of blood plasma = 26.60 g

Mass of ethanol = 0.0488 g

Putting values in above equation, we get:

$\text{Mass percent of ethanol}=\frac{0.0488g}{26.60g}\times 100=0.183\%$

Hence, the mass percent of alcohol in blood is 0.183 %