Help with law of multiple proportions.

Confused with law of constant proportions. Help me!!?

• I am studying the law of constant proportion and the example given in my book is water. The ratio of the mass of the constituents of water is 1:8 right? Does this mean that 9 gram of water will have 1 gram hydrogen and 8 gram oxygen? Does this also mean that 0.9 gram of water will have 0.1 gram of hydrogen and 0.8 gram of oxygen? Then what will be the mass of hydrogen and oxygen in 1 gram of water? If there are two atoms of hydrogen and one of oxygen shouldn't the ratio of 1 gram of water be 2:16? Does the law of constant proportions take into account the number of atoms too?? I am confused with the ratio of the number of atoms of hydrogen and oxygen and the ratio of its masses. I didn't understand the mole concept and Dalton's postulates either. Please help immediately!!

• Answer:

  Law of constant proportions or Proust's law is very simple to understand... first of all, if u think that it takes account of number of atoms, well u r wrong...the Proust's law says that elements combine in definite proportion by "mass" not number.... hydrogen and oxygen combines to form water in ration 1:8 by mass and ur conclusion dat 9g water contains 1g hydrogen and 8g oxygen, 0.9g water contains 0.1g of hydrogen and 0.8g oxygen is true. U can compare the ratios 0.1:0.8 = 1:8 since u cannot calculate the mass of water with TOTAL precision, the mass of hydrogen and oxygen in 1g water will be approximately 0.33g and 0.66g respectively.. Always note that there is always an error of +or- 2...so still the law holds..Actually when u add up 0.33 and 0.66, it'll be 0.99 which is very very close to 1.00.....still u r confused in the concept of 1g of water, u can take help of your Mathematics teacher. Now, about ur argument on ratio=2:16....don't u think 2:16=1:8? Also,,,Dalton's all postulates were not correct.. 1. "All matter is composed of small, indivisible particles called atoms." He said atoms to be indivisible (due to lack of technology he wasn't able to find out at that time). Well, atoms are surely divisible into protons, neutrons & electrons....In ur higher studies you will learn that even protons and neutrons are divisible....Anyway, his postulate suggested that atoms are the smallest unit of matter. 2. "All atoms of a given element are identical in mass and properties." Consider u have a bottle full of oxygen gas. U must be knowing that oxygen is an element with mass 16u, atomic number 8, valency 2- , etc etc....Now think about it...Wont each and every atom of the oxygen gas present in ur bottle will have same properties...this is what Dalton said in this postulate that all atoms of a given element ( say oxygen ) are having same mass and physical and chemical properties. 3. "Compounds are formed by a combination of two or more atoms in definite arrangements in the ratio of small whole numbers." This postulated stated that 2 or more atoms bond together in definite arrangement to for a compound...consider H2O..here hydrogen an oxygen atoms (3 in total) are bonded to give water with a given molecular structure. Also atoms bond in small whole numbers ( which some years later proved that they even bond in large whole number e.g. C16H31O2Na i,e, sodium palmatite) 4. "Atoms are not created, destroyed or converted into other kinds of atoms during chemical reactions. They are simply rearranged into new compounds." This said that atoms are immortal particles which can never be created nor destroyed. They can only be changed into one form from another e.g. conversion of water to ice though, chemical formula - H2O remains same. Additionally, in a chemical reaction, atoms just rearrange themselves into different structures...here also they are not created nor destroyed. Just get this --- 1 mole=6.022 * 10^23 number of anything....quite a larger no. though.....so supose if I have 6.022*10^23 shirts, i can say i have 1 mole shirts...but that's a vry large no. and makes no sense in daily life...but in chemistry since atoms are very very small and large in number, it is very useful.... For mole concept...just remember these formulas for ur exam...u can take help of ur chemistry teacher for detailed information... n=m/M n=N/N' here, n=no. of moles m=given mass M=molar mass N=given no. of particles N'=Avogadro's constant Hope it helped...