What is the mass % of nitrogen in urea, in arginine, and in or-nithine? (b) How many grams of nitrogen can be excreted as urea when 135.2 g of ornithine is produced?

To determine the grams of chromite (FeCr$_2$O$_4$) needed, we will follow these steps:

1. Calculate the mass of lead(II) chromate (PbCrO$_4$) required for 1 kilogram of paint.
2. Convert the mass of PbCrO$_4$ to moles using its molar mass.
3. Use the mole ratio between PbCrO$_4$ and K$_2$CrO$_4$ (from the replacement reaction) to find moles of K$_2$CrO$_4$.
4. Use the mole ratio from the balanced chemical equation to find moles of FeCr$_2$O$_4$.
5. Convert moles of FeCr$_2$O$_4$ to grams using its molar mass.

First, calculate the molar masses:
Molar mass of PbCrO$_4$:
Pb: $207.2 \text{ g/mol}$
Cr: $52.00 \text{ g/mol}$
O: $16.00 \text{ g/mol}$

$$\text{Molar mass of PbCrO}_4 = 207.2 + 52.00 + (4 \times 16.00) = 323.2 \text{ g/mol}$$

Molar mass of FeCr$_2$O$_4$:
Fe: $55.845 \text{ g/mol}$
Cr: $52.00 \text{ g/mol}$
O: $16.00 \text{ g/mol}$

$$\text{Molar mass of FeCr}_2\text{O}_4 = 55.845 + (2 \times 52.00) + (4 \times 16.00) = 223.845 \text{ g/mol}$$