I can't solve this. Please help

Question

Industrially, nitric acid is produced by the Ostwald process, as represented by the following equations:

 

4NH3(g) + 5O2(g)

→

4NO(g) + 6H2O(l)

2NO(g) + O2(g)

→

2NO2(g)

2NO2(g) + H2O(l)

→

HNO3(aq) + HNO2(aq)

What mass of NH₃ (in grams) must be used to produce 4.73 tons of HNO₃ by the Ostwald process, assuming an 80.0 percent yield in each step (1 ton = 2000 lb; 1 lb = 453.6 g)? Enter your answer in scientific notation.

Answer 1

Moles HNO3 desired = 4.73 tons x 2x10³ lbs/ton x 453.6 g/lb x 1 mol/63 g = 6.81x10⁴ moles HNO₃

Moles NH₃ needed = 6.81x10⁴ mol HNO3 x 4 mol NH3/mol HNO3 = 2.72x10⁵ moles NH₃ needed

Three reactions each at 80% yield = 51.2% overall yield. To correct for this, moles NH3 = 2.72x10⁵ mol/0.512 = 5.31x10⁵ moles NH3

Mass NH3 = 5.32x10⁵ mol x 17g/mol = 9.03x10⁶ g NH3