What is the definition of valence number?

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.

The molar mass of iron(fe) is 55.8 g/mol. what is the mass in grams of 2.25 moles of iron?

+= + + balance the equation on coefficients, ex. 1,1,1