Question

What mass of chromium metal is in a sample that contains 6.9 × 1025 formula units of ammonium dichromate, (NH4)2Cr2O7(s)?

Answer 1

Moles of $(NH_4)_2Cr_2O_7=\frac{6.9\times 10^{25}}{6.022\times 10^{23}}=114.57 moles$

Moles of Cr $=2\times 114.57=229.15moles$

Mass of Cr$=229.16mol \times 51.99g/mol=1.2\times 10^4 g$