Question

If you have 150. mg of cysteine, how many h atoms are present in your sample?

Answer 1

Moles (mol) = mass (g) / molar mass (g/mol)

Mass of cysteine = 150 mg = 150 x 10⁻³ g
Molar mass of cysteine = 121.16 g/mol
Moles of cysteine = 150 x 10⁻³ g / 121.16 g/mol
                            = 1.238 x 10⁻³ mol

Cysteine has chemical formula as C₃H₇NO₂S.
Hence, 1 mole of C₃H₇NO₂S produces 7 moles of H atoms.
Hence, moles of H atoms present in the sample = 1.238 x 10⁻³ mol x 7
                                                                                 = 8.666 x 10⁻³ mol

According to the Avogadro's constant 1 mole of substance has 6.022 × 10²³ particles.

Hence,
    number of H atoms in the sample = 8.666 x 10⁻³ mol x 6.022 × 10²³ mol
                                                              = 5.219 x 10²¹