Question

The forensic technician at a crime scene has just prepared a luminol stock solution by adding 10.0 g of luminol into a total volume of 75.0 mL of H2O .What is the molarity of the stock solution of luminol?Express your answer with the appropriate units.

Answer 1

1) List the known and unknown quantities.

Sample: luminol.

Mass: 10.0 g.

Volume: 75.0 mL.

Molarity: unknown.

2) Set the equation.

`M=\frac{moles\text{ }of\text{ }solute}{liters\text{ }of\text{ }solution}`

3) Conversion of units.

3.1-Convert mass to moles.

The molar mass of luminol (C8H7N3O2) is 177.1601 g/mol.

`mol\text{ }C_8H_7N_3O_2=10.0\text{ }g\text{ }C_8H_7N_3O_2*\frac{1\text{ }mol\text{ }C_8H_7N_3O_2}{177.1601\text{ }g\text{ }C_8H_7N_3O_2}`
`mol\text{ }C_8H_7N_3O_2=0.05645\text{ }mol\text{ }C_8H_7N_3O_2`

3.2-Convert mL to L.

1 L = 1000 mL

`L=75.0\text{ }mL*\frac{1\text{ }L}{1000\text{ }mL}=0.075\text{ }L`

4) Plug in the known quantities.

`M=\frac{0.05645\text{ }mol\text{ }C_8H_7N_3O_2}{0.075\text{ }L}`
`M=0.7527\text{ }M\text{ }C_8H_7N_3O_2`

The molarity of the stock solution is 0.75 M C8H7N3O2.

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