Safe Amount Of Arsenic In Drinking Water
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Safe Amount Of Arsenic In Drinking Water

The standard for arsenic content in public water supplies is less than or equal to 10 parts per billion (ppb) arsenic. Assuming that this arsenic is present as arsenate ASO43-,
What mass of sodium arsenate would be present in a 1 L sample of drinking water that just meets the stadard?

The strings:
S7P1A15 (containership - mass).

The math:
Pj Problem of Interest is of type containership (mass).

1 ppb = 1μg/L = (1 x 10-6) g /L
So, 10 ppb = 10 x 10-6 g/L
So, mass of arsenic in 1 L sample of drinking water = 10-5 g.
Molar mass of arsenic = 74.92 g
So mol of arsenic in 1 L = (1/74.92)(10-5) = 0.01335 x 10-5 mol

Na2AsO4 -------> 3Na+ + AsO43-
Mole ratio Na3AsO4:AsO43- = 1:1
So, mol of Na3AsO4 in 1 L sample of drinking water = 0.01335 x 10-5 mol.
Molar mass of Na3AsO4 =3x23 +74.92 + 4x16 = 69 + 74.92 + 64 = 207.92 g
So, mass of Na2AsO4 in 1 L sample of drinking water = 207.92(0.01335)(10-5) = 2.8 x 10-5 g of Na2AsO4.

Math

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