A gaseous compound containing hydrogen and carbon is...

A gaseous compound containing hydrogen and carbon is decomposed and found to contain $85.63 \%$ C and $14.37 \%$ H by mass. The mass of 258 mL of the gas, measured at STP, is 0.646 g. What is the molecular formula of the compound?

Key Concepts

Gas Laws and Molar Volume at STP

At standard temperature and pressure (STP), gases are known to occupy a fixed molar volume. This concept is crucial for converting a given volume of gas to the number of moles, which ties together the observed physical properties (like density) with chemical composition.

Percent Composition

Percent composition represents the proportion by mass of each element in a compound. It is used to determine the relative number of moles of each element present in a sample by converting the mass percentages to mass, and then to moles using their atomic masses.

Empirical Formula Determination

Empirical formula determination involves converting the percent composition into moles for each element and then finding the simplest whole-number ratio. This ratio provides the basic 'recipe' for the compound, showing the lowest number of atoms of each element present.

Molecular Formula Determination

Once the empirical formula is known, the molecular formula is determined by comparing the mass of the empirical formula unit to the actual molar mass of the compound. This involves calculating the empirical formula mass and using additional data, such as the density or measured gas volume, to find the multiple that relates the empirical formula to the actual molecular structure.

Transcript

00:04 This question tells us that a gaseous compound containing hydrogen and carbon is found to have 85 .63 % carbon and 14 .37 % hydrogen by mass.

00:15 And we're told that the mass of a sample of this gas is 0 .646 grams.

00:22 And that sample has a volume of 258 milliliters and is measured at stp.

00:27 So stp stands for standard temperature and pressure, and it means 273 kelvin as our temperature and one atmosphere.

00:34 Sphere as our pressure.

00:36 And they ask us to find the molecular formula of the compound.

00:39 So the first thing that i'm going to do is use the ideal gas equation to find the number of moles of gas in this sample.

00:48 And the reason i want to do that is because then i can use the number of moles and this gram value to get a molar mass of the gas compound.

00:57 And then i can use the percentages to figure out how much of that is carbon and how much of that is hydrogen.

01:04 So the first thing i want to do is use that ideal gas equation.

01:09 Pv equals nrt.

01:11 P is pressure.

01:12 V is volume.

01:13 N is the number of moles.

01:14 That's what we're looking for here.

01:16 R is a constant and t is temperature.

01:19 For my units, i need my pressure to be in atmospheres, which we have, by volume to be in liters, which we have mililiters right now.

01:27 So to get to liters, i'm just going to divide this by 1 ,000.

01:29 So i'll get 0 .258 liters...

Please give Ace some feedback