Formulas 5.1 - Masses of Particles

Question 1

How do you calculate the approximate mass of an atom relative to carbon-12 isotope?

Answer 1

(# of Protons + # of Neutrons ) * 12
12

Question 2

What formula do you use to calculate Relative Atomic Mass from Isotopic Masses and Percentage Abundances?

Answer 2

Question 3

Using the data below, calculate the relative atomic mass of Silver.

Answer 3

A_r = (106.9 * 51.8) + (108.9 * 48.2)
100
= 5537.42 + 5248.98
100
A_r(Ag) = 107.86

Question 4

Using the data below, calculate the relative atomic mass of Hydrogen.

Answer 4

A_r = (1.008 * 99.986) + (2.014 * 0.014) + (3.016 * 0.0001)
100
= 100.786 + 0.028 + 0.000
100
A_r(H) = 1.008

Question 5

What formula is used to calculate the Percentage Abundance of each isotope from the mass spectrum?

Answer 5

Peak Height for each isotope
Total Peak Height = Sum of Peak height for each isotope

then multiply by 100.

Repeat for each Isotope

Question 6

Using the data below, calculate the percentage abundance of each of the three isotopes.

Iso 1: ^2.2Pb

Iso 2: ^2.8Pb

Iso 3: ^{<span>5</span>}Pb

Answer 6

Total Peak Height = (5 + 2.8 + 2) = 10

2. 2 / 10 * 100 = 22%
3. 8 / 10 * 100 = 28%
4. 0 / 10 * 100 = 50%

Question 7

What formula do you use to calculate the percentage abundance of each isotope from the mass spectrum?

Answer 7

Step 1: Abundance of the Lighter Isotope
Ar (Relative Isotopic mass)
= ( (Relative Isotopic mass Light * x) + (Relative Isotopic mass Heavy * (100 - x) ) / 100

= x%

Step 2: Abundance of the heavier isotope = 100 - x%

Question 8

The element Litium has two isotopes:

⁶Li with a relative isotopic mass of 6.02

⁷Li with a relative isotopic mass of 7.02

The relative atomic mass of lithium is 6.94. Calculate the percentage abundance of the lighter isotope.

Answer 8

6.94 = ( 6.02x + (7.02 * (100 - x) )

100

6.94 = 6.02x + 702 - 7.02x

100
(x 100 each side) 694 = 6.02x + 702 - 7.02x
(Align x’s on one side) 694 - 702 = 6.02x - 7.02x
(Simplify) -8 = -1x
(x -1 each Side) x = 8%

Question 9

What formula do you use to calculate the relative Molecular Mass of Molecules?

Answer 9

M_r = (# of Atoms * Relative Atomic Mass from the periodic table) + (# of Atoms * Relative Atomic Mass from the periodic table)

Question 10

Calculate the Relative Molecular masses of

ammonia (NH₃)

Answer 10

A_r(N) = 14.0 A_r(H) = 1.0
1 x N Atom 3 x H Atom
M_r = (1 * A_r(N)) + (3 * A_r(H)
M_r = (1 * 14) + (3 * 1)
M_r = 14 + 3
M_r = 17

Question 11

What formula do you use to calculate the relative formula mass of ionic compounds?

Answer 11

Mr = (# of Atoms * Relative Atomic Mass from the periodic table) + (# of Atoms * Relative Atomic Mass from the periodic table) etc.

Question 12

Calculate the Relative Molecular masses of
sodium carbonate (Na2CO3)

Answer 12

A_r(Na) = 23.0 A_r(C) = 12.0 A_r(O) = 16.0
2 x Na Atom 1 x C Atom 3 x O Atom
M_r = (2 * A_r(Na)) + (1 * A_r(C)) + (3 * A_r(O))
M_r = (2 * 23.0) + (1 * 12.0) + (3 * 16.0)
M_r = 46.0 + 12.0 + 48.0
M_r = 106.0 (Don’t forget the .0 at the end)