Now you should see how moles are related to mass, density, and number of molecules or atoms. The density of ethanol is r = 0.789 g/mL. To know the density for a given substance. Density is both temperature and pressure dependent both need to be specified This is definedĪs density (denoted by the Greek letter rho, r) and is in units of g/mL. To answer this we will need a volume to mass conversion factor. How many molecules of ethanol are in 1.0 L of ethanol at 1 atm and 25 oC? This also means that for each mole of ethanol there are two moles of carbon. In each molecule of ethanol there are two carbon atoms. Notice that there is an extra conversion in this equation. How many carbon atoms are in 89.0 g of ethanol (CH 3CH 2OH)?Īgain, we used the same gram to mole conversion factor, the molecular mass, and also Avogadro's number to go from moles to atoms. Also, remember to use the correct number of significant figures. This is why using units is very imporatant. Avogadro's number can be used to cancel out the moles and leaves molecules of ethanol units. In order to cancel units, multiply by the reciprocal of the molecular mass. This can be used to cancel out the given number's units. Now we need to come up with conversion factors to cancel units and get to the desired set of units. We want molecules of ethanol to answer the question. So in this example 89.0 g of ehtanol are given. Use conversion factors to cancel out units and get to your final units.Next determine what you are trying to solve and what units it will be in.Start by writing the given information, including the units.Here are a few hints to use in solving this type of problem: How many molecules of ethanol are in 89.0 g of ethanol? A mole of anything is 6.022 x 10 23 of that item.Ī mol of ethanol molecules = 6.022 x 10 23 ethanol moleculesĪ mol of eggs = 6.022 x 10 23 eggs Example 1. A dozen of anything is 12 of that item:Ī dozen ethanol molecules = 12 ethanol molecules When thinking about a mole, think of it like a dozen. This means that in 1 mol of ethanol, thereĪre 46 g, thus the term molecular mass and molar mass are often used interchangeably. In this case,Įthanol's molecular mass is 46 g/mol. The molecular mass of a molecule is measured in g/mol. This is a lot easier than counting out all those molecules. Therefore we only need to measure 46 g of ethanol to get 6.022 x 10 23 molecules or 1 mol of ethanol. ![]() How does this help us measure out 6.022 x 10 23 molecules of ethanol? As we said before, This means that in 12 g of C there are exactly 6.022 x 10 23 atoms of carbon.Įach atom of C has a mass of 12 amu's, which means that: Measure out one mole of ethanol, but how do we do that? Let's redefine a mole.Ī mole is any item that contains the same number of that item as the number of atoms in exactly 12 g ofġ2C. Since 6.022 x 10 23 ethanol molecules are needed, we simply need to Even if a billion people worked together to count out the molecules you wouldn't be able to finish even close to (It would take almost 2 x 10 16 years to do this. Besides you probably wouldn't live long enough to finish. But this is far too time consuming and expensive. If you didn't know better, you might think that you had to somehow count out all of these molecules individually. Imagine that you are a chemist and you would like to measure out 6.022 x 10 23 molecules of ethanol. ![]() Avogadro's number, denoted by the symbol N A is 6.022 x 10 23.įine, but what does this have to do with chemistry? That's a fair question. What is a mole? Basically a mole is defined as Avogadro's number ofĪnything. Using the density to relate volume to number of moles.Converting sample mass to the number of molecules or atoms.Determining the number of moles in a given mass of a sample.The interconversion between mass, moles, number of atoms or molecules, and volume is described. ![]() Overview: In this module, we introduce the mole, a unit of measure that is usedĮxtensively in chemistry.
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