Problems with stoichiometry?

Stoichiometry Problems?

• i am having problems with stoichiometry problems XDXD i need some help on how to figure them out cause i have no idea how to *screams* please please please help me due-26/01/09 ...show more

• Answer:

  I don't rally understand what you want; but some time ago, I prepared a general answer on the concept of the mole; that might help. Here is the earlier answer (rather long, I'm afraid): THE MOLE Chemical reactions take place on a molecular basis. A molecule of one substance reacts with a given number of molecules of another (eg 2 molecules of hydrogen react with one molecule of oxygen to produce 2 molecules of water; given by the formula 2H2 + O2 → 2H2O). However, a molecule is very tiny, and it is inconvenient to consider reactions in terms of individual molecules. But we need to be able to calculate how much of a given substance will react with how much of another (eg; how many grams of hydrogen will react with a given weight of oxygen in the above reaction, and how much water would be produced.) All elements are made of atoms composed of protons, neutrons and electrons. All protons neutrons and electrons are the same for every element (ie, there is no difference between the proton in an oxygen atom and that in a hydrogen atom). But each atom contains a different number of protons. Hydrogen atoms contain 1, oxygen atoms contain 8. The number of protons determines an element's position in the periodic Table (eg, Hydrogen is element 1, Oxygen is element 8). As well as protons; the atoms also contain neutrons in their nucleus. Usually the number of neutrons is the same as the number of protons; but not always - especially for the heavier elements; which may have slightly more or slightly less neutrons than protons. Neutrons weigh nearly the same as protons, so if we want to compare the weights of atoms; we only need to count the number of protons and neutrons (the electrons make only a tiny difference). Giving the proton a nominal value of one, the 'atomic mass' can then be calculated. For hydrogen, the 'atomic mass' is 1 (one proton only, no neutrons); for oxygen it is 16 (8 protons and 8 neutrons) For some elements, the figure is not a whole number, because the number of neutrons might vary (Magnesium for instance is element 12, but but some of the atoms have 13 neutrons, so on average there are slightly more neutrons than 12, so its atomic mass is 24.31) Now using these atomic masses, we can calculate the masses of materials in a chemical reaction. For instance, we know Hydrogen is element 1, so an H2 molecule will have mass 2, and in the above reaction, 2 molecules of Hydrogen will have mass 4. These will react with 1 molecule of O2, having mass 32 (two atoms each of atomic mass 16). By convention, the average atomic mass is converted to grams. This is called a 'gram atom', or 'Mole'. Thus one mole of oxygen (O2) will contain 32 grams of oxygen. There are a very large number of molecules in a mole (6.022X10^23), so it is a lot easier to calculate chemical reactions using the number of grams in a mole. Therefore, in our reaction between Hydrogen and Oxygen, we know that 4 grams of Hydrogen will combine with 32 grams of Oxygen to produce 36 grams of water. Remember that for some elements, the weight of a mole is not an even number (as for Magnesium). For any molecule, the weight of a mole can be calculated by adding up all the atomic masses, and converting to grams. Eg, for Fe2O3, the answer is 55.85*2 + 16*3 = 159.7 Therefore, a mole of Iron (ferrous) oxide will weigh 159.7 grams