Question

Using modern analytical techniques, it is possible to detect sodium ions in concentrations as low as 75 pg/mL

A.What is this detection limit expressed in molarity of Na+?

Express your answer using two significant figures.

B. What is this detection limit expressed in Na+ ions per cubic centimeter?

Express your answer using two significant figures

Answer 1

Part A : detection limit = 3.3 x 10^-9 M

Part B : detection limit = 2.0 x 10¹² ions/cm³

Explanation

Part A : Let volume of solution = 1 mL

mass Na = (concentration Na⁺) * (volume of solution)

mass Na = (75 x 10^-12 g/mL) * (1 mL)

mass Na = 75 x 10^-12 g

moles Na = (mass Na) / (molar mass Na)

moles Na = (75 x 10^-12 g) / (23.0 g/mol)

moles Na = 3.26 x 10^-12 mol

molarity Na = (mass Na) / (volume of solution in Liter)

molarity Na = (3.26 x 10^-12 mol) / (1 x 10^-3 L)

molarity Na = 3.3 x 10^-9 M