Question

. Chromium 6, that is compounds/ions where chromium has a +6 chargeare known to be human carcinogens. The legal limit for chromium 6 indrinking water is established as 10 ppb in California. If a glass of water(250 mL) contains 10 ppb chromium 6 how many chromium atoms arein the glass?

Answer 1

ANS. 1 ppb is a unit of concentration and is defined as part per billion.

Formula is given as :

Concentration(ppb) = mass of solute(gm)/ mass of solvent * 10⁹

Given that volume of water is 250mL.

Density of water is nearly 1gm/mL

So mass of 250mL = 250gm

We have to calculate mass of solute that is cromium ions.

So

10 = mass of solute/250gm * 10⁹

MASS of solute = 10*250*10^-9

= 2500 *10^-9

= 2.5*10^-6 gm

Atomic mass of chromium is 52.Charge does not affect the atomic mass of an element since neutrons and protons remain constant.So i will calculate for chromium only for convenience.

52 gm of chromium contains = 6.626*10²³ atoms (avogadro number)

1 gm of chromium will contain = 6.626/52 *10²³ atoms

2.5 *10^-6gm of chromium will contain = 6.626/52 * 2.5 * 10¹⁷

= 0.318 *10¹⁷

= 3.2 *10¹⁶ atoms of chromium or chromium VI