- 11.5g of H2 gas are placed in a balloon at STP. Determine the volume of the balloon?
In such a scenario, one that you may encounter on any given day, you are being asked to convert between two units of factors with no direct relation (Mass cannot be converted in a single step into Volume; they represent separate measurements). Therefore, it is necessary to follow this convenient "road map":
This is simplified by the following points:
- Mass to Moles, and vice versa, uses Molar Mass - Atomic Mass of Element or Compound g/mol
- Volume (at STP) to Moles, and vice versa, uses Molar Volume - 22.4 L/mol
- Particles (Molecules/Atoms) to Moles, and vice versa, uses Avogadro's Number - 6.02 x 10^3
Using such information, one can solve the aforementioned problem as follows:
11.5 g (always begin with the given number) x 1 mole/ 2 grams = 5.75 moles
5.75 moles x 22.4 Liters/ 1 mole = 129 Liters
It is important that all the data and units are inserted and cancelled properly and for that, here's another "road map".
And here is another example:
- A sample of oxygen gas contains 3.5 x 10^21 molecules. How many grams of oxygen is this?
3.5 x 10^21 molecules x 1 mole/ 6.02 x 10^23 molecules
=5.8 x 10^-3 moles
5.8 x 10^-3 moles x 2 (16.0) g */ 1 mole
=0.19 grams of oxygen gas
*Pay attention to isolating diatomic elements as their atomic mass becomes that of two molecules of that element*
The universality of the application of multi-step conversions, in that it is common in dimensional analysis and recurring in chemistry, is made even simpler by this video:
