# Mass Ratios and Mutiple Proportions

1. Sep 22, 2005

### Soaring Crane

The "Law of Multiple proportions" states that if two elements combine in different ways to form different substances, the mass ratios are small, whole number multiples of each other. Suppose that you have 66g of carbon combined with oxygen in carbon dioxide (CO2), how many grams of oxygen would you expect to be involved?

Well, I looked at the atomic mass ratios of oxygen to carbon, or 32.00 to 12.01. By dividing this I get 8/3, so I multiply (8/3)*66 g for 176 g of O? Is this correct?

Oh, exactly how would I express this in significant figures (or the proper form)? 1.8*10^2 g (if this is correct)?

Thanks.

Last edited: Sep 22, 2005
2. Sep 22, 2005

### bomba923

"66 grams" implies two significant figures. The ratio of carbon to oxygen in pure carbon dioxide is 1 atom carbon : 2 atoms oxygen . So,

$$\left( {\frac{{66g\;{\text{C}}}}{1}} \right)\left( {\frac{{1{\text{ mole C}}}}{{12g\;{\text{C}}}}} \right)\left( {\frac{{{\text{2}}\;{\text{moles O}}}}{{1{\text{ mole C}}}}} \right)\left( {\frac{{{\text{16}}g\;{\text{O}}}}{{1{\text{ mole O}}}}} \right) = 176g\;{\text{O}} \Rightarrow \boxed{{\text{180g}}\;{\text{O}}}$$

Just 180 g is fine I think

Last edited: Sep 23, 2005