Bromine has two naturally occurring isotopes $(\mathrm{Br}-79$ and Br-$81$) and an atomic mass of $79.904$ amu.

(a) If the natural abundance of $\mathrm{Br}-79$ is $50.69 \%$, what is the natural abundance of Br-$81$?

(b) If the mass of Br-$81$ is $80.9163 \mathrm{amu}$, what is the mass of Br-$79$?

The mass of a ${ }_3^7 \mathrm{Li}$ atom is $7.01600 \mathrm{amu}$. Calculate its mass defect.

Determine the number of atoms present in $8.6 \text{ mg}$ of Ca? The atomic weight of Ca is $40.08\text{ amu}$.

There are two stable isotopes of iridium: ${ }^{191} \mathrm{Ir}(190.96 \mathrm{amu})$ and ${ }^{193} \mathrm{Ir}(192.96 \mathrm{amu})$. If you were to randomly pick an iridium atom from a large collection of iridium atoms, which isotope are you more likely to select?

Determine the molecular formula of a compound with an empirical formula of $\mathrm{NH}_2$ and a formula mass of 32.06 amu.

Silicon has three naturally occurring isotopes (Si-28, Si-29, and Si-30). The mass and natural abundance of Si-28 are 27.9769 amu and 92.2%, respectively. The mass and natural abundance of Si-29 are 28.9765 amu and 4.67%, respectively. Find the mass and natural abundance of Si-30 .

The element rubidium has two naturally occurring isotopes. The atomic mass of ${ }^{85} \mathrm{Rb}(72.17$ percent abundant) is $84.911794 \mathrm{amu}$. Determine the atomic mass of ${ }^{87} \mathrm{Rb}$ (27.83 percent abundant). The average atomic mass of $\mathrm{Rb}$ is $85.4678 \mathrm{amu}$.

What is the mass in grams of 6.022 $\times 10^{23}$ O-atoms of mass 16.00 amu?

Calculate the number of atoms in 2.0 g of hydrogen atoms. The atomic mass of hydrogen is 1.01 amu.

Calculate the atomic mass of copper given the following data:

$$\begin{array}{clc} \text { Isotope } & \text { Mass } & \text { Abundance } \\ \hline{ }^{63} \mathrm{Cu} & 62.930 \mathrm{amu} & 69.09 \% \\ { }^{65} \mathrm{Cu} & 64.928 \mathrm{amu} & 30.91 \% \end{array}$$

The atomic weight of boron is reported as 10.81, yet no atom of boron has the mass of 10.81 amu. Explain.

In this exercise, perform the indicated calculations by first expressing all numbers in scientific notation.

One atomic mass unit (amu) is $1.66 \times 10^{-27} \mathrm{~kg}$. If one oxygen atom has 16 amu (an exact number), what is the mass of $125,000,000$ oxygen atoms?

Naturally occurring uranium consists of two isotopes, whose masses and abundances are shown below.

$$\begin{aligned}{} &\text { Isotope }& \text { Abundance }&\text { Mass }\\ &{ }^{235} \mathrm{U}& 0.720 \% & \quad 235.044 \mathrm{amu}\\ &{ }^{238} \mathrm{U} & 99.275 \%&\quad 238.051 \text { amu }\\ \end{aligned}$$

Gallium has only two naturally occurring isotopes, Ga- $69$ and Ga- $71$. The mass of Ga- $69$ is $68.9256$ amu, and the mass of Ga- $71$ is $70.9247$ amu. Use the atomic mass of gallium to determine the relative abundance of each isotope in a naturally occurring sample.