Conversions & ppb - Arsenate in Drinking Water

[uestions]

Homework Statement

The new standard for arsenate in drinking water, mandated by the Safe Drinking Water Act, requires that by January 2006, public water supplies must contain no greater than 10 parts per billion (ppb) arsenic. Assuming that this arsenic is present as arsenate, AsO₄^3-, what mass of sodium arsenate would be present in a 1.00-L sample of drinking water that just meets the standard?

Homework Equations

molar mass of sodium arsenate, Na₃AsO₄, = 207.89 g

The Attempt at a Solution

(10 moles of Na₃AsO₄/10⁹moles of H₂O) * (1 mole H₂O/18 g) * (1000 g/ 1 L) * (207.89 g/ 1 mole Na₃AsO₄) = 1.2 * 10^-4 g of Na₃AsO₄ in 1 L of water

I did conversions from the ratio of moles of sodium arsenate and water to grams of sodium arsenate per liter of water. However, this answer is incorrect according to my textbook's answer, which is 2.8 * 10^-5 g of Na₃AsO₄ in 1 liter of water.
Please help me understand what I am doing wrong, or what I'm misinterpreting.

 

[Chestermiller]

When they say that arsenic is present at 10 ppb in aqueous solution, what they mean is that there are 10 grams of arsenic present in every billion grams of water.

Chet

 

[Borek]

Note: ppb, ppm notation is often ambiguous, unless it is clearly stated what it is intended to mean (w/w, v/v. atoms/atoms, whatever). If nothing is explained it typically means w/w (weight/weight), but you should be always vigilant.