why percentage yield is never 100%

competing reactants

reactions that occur along with the principal reaction involving the reactants and/or the products of a principal reaction

slow reactions

if you measure the amount of product too early, the reaction may not have gone to completion

equipment used

if the reaction vessel is pitted, rough, has a dirty surface or an odd shape it will decrease the rate of at which the products are being formed and thus you may measure the products too early

purity of the reactants

we assume that all reactants are 100% pure, when they are never pure

greater than 100%

if the measured product of the reaction contains impurities tat cause it’s mass to be greater than it actually would be if the product was pure