Question

chemistry
worksheet 2c
isotopes and average atomic mass

1. northstarium (ns) has three naturally - occurring isotopes:

^44ns 74.655%
^46ns 24.958%
^48ns 0.387%
calculate the average atomic mass of northstarium (ns)

1. in a given sample of bromine, 20% of the atoms are ^81br atoms, and 80% are ^79br. what is the aam of this bromine sample?
2. rubidium has two common isotopes, ^85rb and ^87rb. if the abundance of ^85rb is 72.2% and the abundance of ^87rb is 27.8%, what is the average atomic mass of rubidium?

multiple choice

1. isotopes are atoms of the same element that have different

a. numbers of protons
b. masses
c. numbers of electrons

1. the atomic number of oxygen, 8, indicates that there are eight

a. protons in the nucleus of an oxygen atom
b. neutrons outside the oxygen atoms nucleus
c. energy levels in the oxygen atoms nucleus
periodic table of the elements
new jersey department of education

Explanation:

Step1: Recall average atomic mass formula

The formula for average atomic mass (AAM) is $AAM=\sum_{i = 1}^{n}(m_i\times p_i)$, where $m_i$ is the mass of the isotope and $p_i$ is the percentage abundance of the isotope (expressed as a decimal).

Step2: Solve problem 1

Let the masses of the isotopes of Northstarium ($^{44}Ns$, $^{46}Ns$, $^{48}Ns$) be 44, 46 and 48 respectively. The abundances as decimals are $p_1 = 0.74655$, $p_2=0.24958$, $p_3 = 0.00387$.
$AAM=(44\times0.74655)+(46\times0.24958)+(48\times0.00387)$
$=32.8482 + 11.48068+0.18576$
$=44.51464$

Step3: Solve problem 2

For bromine, let the mass of $^{81}Br$ be 81 and $^{79}Br$ be 79. The abundances as decimals are $p_1 = 0.2$ and $p_2=0.8$.
$AAM=(81\times0.2)+(79\times0.8)$
$=16.2+63.2$
$=79.4$

Step4: Solve problem 3

For rubidium, let the mass of $^{85}Rb$ be 85 and $^{87}Rb$ be 87. The abundances as decimals are $p_1 = 0.722$ and $p_2=0.278$.
$AAM=(85\times0.722)+(87\times0.278)$
$=61.37+24.186$
$=85.556$

Step5: Solve multiple - choice 1

Isotopes are atoms of the same element that have different masses. They have the same number of protons and electrons, but different numbers of neutrons which causes the mass difference. So the answer is b.

Step6: Solve multiple - choice 2

The atomic number of an element is equal to the number of protons in the nucleus of an atom of that element. So for oxygen with atomic number 8, there are 8 protons in the nucleus of an oxygen atom. The answer is a.

Answer:

1. Average atomic mass of Northstarium: 44.51464
2. Average atomic mass of bromine sample: 79.4
3. Average atomic mass of rubidium: 85.556

Multiple - choice 1: b. masses
Multiple - choice 2: a. protons in the nucleus of an oxygen atom