text: what is the mass in grams of hcl when 6l of molecular hydrogen measured at stp with excess of molecular cl gas balanced equation: h2(g)+cl2(g)--->2hcl(g) i understand your question to mean the following: what is the mass in grams of hcl produced when 6l of molecular hydrogen measured at stp (1 atm and 273.15 k) reacts completely with excess of molecular cl gas?  pv = nrt  n = pv/rt  t = 273.15 k p = 1 atm r = 0.08206 l atm/mol k v = 6l  n = (1 atm x 6l)/ (0.08206 l atm/mol k x 273.15k)  n = 0.26768 mol h2 --------------------------------------------------------------------------- from the chemical equation h2(g)+cl2(g)--->2hcl(g)  (2 mol hcl/1 mol h2) 0.26768 mol h2 = 0.53536 mol hcl  ----------------------------------------------------------------------------- hydrogen atomic weight = 1.00794 chlorine atomic weight = 35.453  thus, hcl molecular weight = 36.46094 g/mol --------------------------------------------------------------------------------  weight of hcl = 0.53536 mol hcl x 36.46094 g/mol = 19.52 g