7. Of the balanced chemical equations below, which one most correctly represents a combustion reaction?
a. CaO + H20 Ca(OH) 2
b. CH4 + 202 CO2 + 2H2O
c. 2 Mg + O202 Mgo
d. 2 N2 + 3H22 NH 3

The content of manganese (mn) in steel was determined spectrophotometrically and with the use of the standard addition method. an unknown sample of mn from a digested steel sample gave an absorbance of 0.185 when analyzed spectrophotometrically. when 5.00 ml of solution containing 95.5 ppm mn was added to 50.0 ml of the unknown steel solution (digested sample), the absorbance was 0.248. calculate the concentration, in parts-per-million (ppm), of mn in the digested steel sample solution.