# chem

posted by on .

The combustion of hydrogen–oxygen mixtures is used to produce very high temperatures (approximately 2500 °C) needed for certain types of welding operations. Consider the reaction to be
H2(g) + 1/2O2(g) --> H20(l)

Standard enthalpy is -241.8kj

What is the quantity of heat evolved,
in kilojoules, when a 140g mixture containing equal parts of H2 and O2 by mass is burned?

• chem - ,

Please don't change your screen name when posting different questions. It gets us confused.
This is a limiting reagent problem as well as a thermodynamic one. First, determine the limiting reagent. I didn't work through all of the problem but I think the limiting reagent is oxygen. Check me out on that. Calculate how many mols H2O are formed from the limiting reagent. The reaction produces 241.8 kJ/1 mol H2O so mols x kJ/mol should give you the answer. (I divided the 140 gram mixture into 70 g hydrogen and 70 g oxygen since the problem said equal parts by mass.)

• chem - ,

yaya that's right, lol assuming u neeed it for mastering chemistry, but i don't get why you need to use the moles of H20.

• chem - ,

you don't have to, you can just use the moles of the limiting reagent and multiply it by 241.8kJ/mol