Formulas-part+1

//** Relative Atomic Mass **//

//“Relative atomic mass is the mass of an element on a scale where carbon-12 has a mass of 12g”//. What this basically means is that we measure the mass of all elements by comparing them with the mass of carbon 12.

Lets suppose we break down a carbon-12 atom into 12 equal parts. So every one part will be one mass unit. Therefore each one unit is used as a measurement for measuring the mass of an atom (just like we use the kilogram as a unit of measurement).



So:


 * Hydrogen consists of only one of these units and it has a mass of 1 units.
 * Magnesium has 24 of these units and it has a mass of 24.

And so on….

The relative atomic mass is denoted by A r

If you look in the periodic table provided on page 6 of the IB data booklet you will see the relative atomic masses of all elements given to two decimal places.

//**Relative Molecular Mass **//

//” Relative molecular mass is the sum of the relative atomic masses of all the atoms in a molecule.” // So the relative atomic mass of water (H 2 O) is the sum of the following:

(Relative atomic mass of Hydrogen) x 2 + (relative atomic mass of Oxygen)

= 2 x (1.01) + (16.00)=18.02

The relative molecular mass is denoted by M r

//Basically the mass of one mole of an element is the relative atomic mass in grams!! //

//(referring to the periodic table in the data booklet) //


 * 1 mole of H atoms __weighs 1.01 grams__
 * <span style="font-family: Tahoma,sans-serif; font-size: 14pt;">1 mole of He atoms __weighs 4.00 grams__
 * <span style="font-family: Tahoma,sans-serif; font-size: 14pt;">1 mole of Li atoms __weighs 6.94 grams__

<span style="font-family: Tahoma,sans-serif; font-size: 14pt;">And so on………..

<span style="font-family: Tahoma,sans-serif; font-size: 14pt;">From what we learnt above you can conclude that:

<span style="font-family: Tahoma,sans-serif; font-size: 14pt;">"The mass of one mole of a molecule is the relative molecular mass in grams" <span style="font-family: Tahoma,sans-serif; font-size: 14pt;">Example: 1 mole of water molecules weigh 18.02 grams

<span style="font-family: Tahoma,sans-serif; font-size: 14pt;">The following formula can be used to calculate the number of moles given the mass of a compound and the molar mass:



//**<span style="color: #ff009b; font-family: Tahoma,sans-serif; font-size: 14pt;">(Molar mass is the mass of one mole of a substance) **//

<span style="font-family: Tahoma,sans-serif; font-size: 14pt;">Let’s use a few examples in order to clarify this:

//<span style="font-family: Tahoma,sans-serif; font-size: 14pt;">Example1: //

<span style="font-family: Tahoma,sans-serif; font-size: 14pt;">Calculate the number of moles of each of the following?

<span style="font-family: Tahoma,sans-serif; font-size: 14pt;">(a) 20g of Ca

<span style="font-family: Tahoma,sans-serif; font-size: 14pt;">(b) 10g of H 2 SO 4

<span style="font-family: Tahoma,sans-serif; font-size: 14pt;">(c) 50g of CaCO 3

<span style="font-family: Tahoma,sans-serif; font-size: 14pt;">(d) 5g of C 6 H 12 O 6
 * <span style="font-family: Tahoma,sans-serif; font-size: 19px;">(a) we know that the molar mass of calcium is 40.08g

<span style="font-family: Tahoma,sans-serif; font-size: 19px;"> || <span style="font-family: Tahoma,sans-serif; font-size: 14pt;">(b) The molar mass of H 2 <span style="font-family: Tahoma,sans-serif; font-size: 14pt;">SO 4 <span style="font-family: Tahoma,sans-serif; font-size: 14pt;">= (molar mass of H) x 2 + (molar mass of S) + (molar mass of O) x 4 <span style="font-family: Tahoma,sans-serif; font-size: 14pt;">= (1.01x2)+(32.06)+(4x16)= 98.08 g mol -1 <span style="font-family: Tahoma,sans-serif; font-size: 14pt; vertical-align: super;"> || <span style="font-family: Tahoma,sans-serif; font-size: 14pt;">= (40.08)+(12.01)+(16x3)= 100.09 g mol -1 || <span style="font-family: Tahoma,sans-serif; font-size: 14pt;">(d) molar mass of C 6 <span style="font-family: Tahoma,sans-serif; font-size: 14pt;">H 12 <span style="font-family: Tahoma,sans-serif; font-size: 14pt;">O 6 <span style="font-family: Tahoma,sans-serif; font-size: 14pt;">= (12.01x6)+(1.01x12)+(16x6)= 180.18 g mol -1 <span style="font-family: Tahoma,sans-serif; font-size: 14pt; vertical-align: super;"> ||
 * <span style="font-family: Tahoma,sans-serif; font-size: 14pt;">(c) Molar mass of CaCO 3

//<span style="font-family: Tahoma,sans-serif; font-size: 14pt;">Example2: // <span style="font-family: Tahoma,sans-serif; font-size: 14pt;">What is the mass of each of the following?

<span style="font-family: Tahoma,sans-serif; font-size: 14pt;">(a) 4 mol of hydrogen atoms <span style="font-family: Tahoma,sans-serif; font-size: 14pt;">(b) 4 mol of H 2 molecules <span style="font-family: Tahoma,sans-serif; font-size: 14pt;">(c) 0.5 mol of CO 2

<span style="font-family: Tahoma,sans-serif; font-size: 14pt;">Number of moles(n)=4 mol <span style="font-family: Tahoma,sans-serif; font-size: 14pt;">Molar mass of hydrogen(M)=1.01 <span style="font-family: Tahoma,sans-serif; font-size: 14pt;">Mass (m)=? <span style="font-family: Tahoma,sans-serif; font-size: 14pt;">So m=4x1= 4g || <span style="font-family: Tahoma,sans-serif; font-size: 14pt;">(b)The difference between this and the previous question is that this question talks about a hydrogen molecule that is H 2 while in the previous one we talked of a hydrogen atom (H) only. <span style="font-family: Tahoma,sans-serif; font-size: 14pt;">Molar mass of H2=2x1.01=2.02g <span style="font-family: Tahoma,sans-serif; font-size: 14pt;"> m= 4x2.02=8.08g || <span style="font-family: Tahoma,sans-serif; font-size: 14pt;">Molar mass=(12.01)+(16x2)=44.01g <span style="font-family: Tahoma,sans-serif; font-size: 14pt;"> m=44.01x0.5=22.005g ||  || <span style="color: #05377f; font-family: Tahoma,Geneva,sans-serif; font-size: 120%;">Well done!!
 * cccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccc || ccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccc ||
 * <span style="font-family: Tahoma,sans-serif; font-size: 14pt;">(a) here you just need to rearrange the formula
 * <span style="font-family: Tahoma,sans-serif; font-size: 14pt;">(c)

By the end of this unit you should be able to:
 * Define the terms //'relative atomic mass'// and //'relative molecular mass'//
 * Calculate the number of moles, the mass or the molar mass of a substance using the given equation

If you feel comfortable with this, then move on to the next unit.

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