Let’s practice converting between mass,

moles, molecules, and atoms. Remember that we can convert grams into moles

by using the molar mass, which in units of grams per mole. We can convert moles into molecules or atoms

or particles, using Avogadro’s Number, and if we have molecules we can break that down

into smaller pieces – like atoms. Number 1 – how many moles are in 0.0265

grams of magnesium chloride? To convert from grams to moles we’re going

to need the molecular weight of magnesium chloride, to do that we will use the periodic

table. So let’s find out the mass of a magnesium

– according to the periodic table – is 24.30 grams per mole, and the mass of a chloride

atom is 35.45 grams per mole, and our formula 0 magnesium chloride has 2 chlorine atoms. So adding up the mass of each element in magnesium

chloride will give us 95.20 grams per mole for magnesium chloride. So, converting we have .0265 grams times. We want grams on the bottom and moles on the

top, so 1 mole over 95.20 grams of magnesium chloride. Notice that our grams of magnesium chloride

will cancel out, and after doing the math we get 2.78 times 10 to the minus 4 moles

of magnesium chloride. Number 2 – what is the mass of 8.15 times

10 to the 19 molecules of carbon dioxide? In order to convert molecules into grams,

we first have to convert to moles. So let’s start by using Avogadro’s Number. There are 6.022 times 10 to the 23rd molecules

of carbon dioxide in 1 mole of carbon dioxide. And our molecules of CO2 would cancel out. And if we stop here, we would have moles of

CO2. We want to continue because we’re looking

for the mass of CO2. So to get the grams of CO2 we will need to

know what the mass of 1 mole of CO2 is, which we can get from the periodic table. So we know that carbon has a mass of 12 grams

per mole, and oxygen has a mass of 16 grams per mole, which we have 2 of. And if we add, we get 44 grams per mole for

carbon dioxide. Plugging in the molar mass of carbon dioxide

and canceling out our moles leaves use with grams, and we end up with 5.95 times 10 to

the minus 3 grams of carbon dioxide. Finally – how many atoms of oxygen are present

in 619 grams of C12H22O11, which by the way is sucrose – or table sugar. In order to convert from grams to atoms of

oxygen we have to first convert to moles, then to molecules, and then into atoms. So let’s start with our 619 grams of C12H22O11,

and in order to convert that into moles we’re going to need molecular weight, which we can

find using the periodic table. So let’s come over here. We know carbon is 12 grams per mole and we

have 12 of those. Hydrogen is 1 gram per mole; we have 22 atoms

of hydrogen. And oxygen is 16 grams per mole, and we have

11. So the molecular weight of C12H22O11 is 342

grams per mole. And if we cancel we have converted to moles. Next we will want to convert the moles into

molecules using Avogadro’s Number. So 1 mole on the bottom, so the moles cancel,

and Avogadro’s Number – 6.022 times 10 to the 23rd molecules of sugar in 1 mole of

sugar. We cancel that out, and we have converted

to molecules of C12H22O11, but we want to go one step further. We’re looking for how many atoms of oxygen

that is, and so, in 1 molecule of C12H22O11 there are 11 atoms of oxygen. And that just comes from the formula. Cancel out molecules and we’re left with

atoms of oxygen, and we get 1.20 times 10 to the 25th atoms of oxygen.