What mass of chromium is in $\pu{100 mL}$ of $\pu{0.0400 mol/L}$ $\ce{(NH4)2Cr2O7}$?
The specific concept is how to find the mass of a certain ion/element in a substance, given the concentration. I am at a total loss to this.
At first I tried to find the molar mass of the chromium itself, which resulted in roughly $\pu{104 g/mol}$. I then tried to find the amount of moles in the concentration, which is
$$\pu{0.1 L} \cdot \pu{0.04 mol/L} = \pu{0.004 mol}$$
Then I multiplied the molar mass of chromium from the amount of moles in the concentration, and I got $\pu{0.208 g}$.
This is not the answer. The answer was $\pu{0.800 g}$
I then tried a different approach. I tried to find the molar mass of my original substance, then divided the molar mass of chromium from the molar mass of ammonium chromate, which will give me the amount of chromium per ammonium chromate. Then I thought all I needed to do was to multiply it by the concentration. This just messed up everything and I was back to square one.
How do I approach this problem?