Question

1. choose the best answer. use significant figures to solve. rubidium is known to have two isotopes—⁸⁷rb and ⁸⁵rb. the abundance of ⁸⁵rb is 72.1. the average atomic mass is ____. 86.5 amu 85.31 amu 55.25 amu 85 amu 12) choose the best answer. what is the average atomic mass of titanium based on the following five common isot ⁴⁶ti ⁴⁷ti 7.44% ⁴⁸ti 73.72%

Explanation:

Step1: Determine the abundance of $^{87}$Rb

Since the abundance of $^{85}$Rb is 72.1%, the abundance of $^{87}$Rb is $100 - 72.1=27.9\%$.

Step2: Use the average - atomic - mass formula

The average atomic mass $A$ of an element with isotopes is given by $A = m_1p_1 + m_2p_2$, where $m_1$ and $m_2$ are the masses of the isotopes and $p_1$ and $p_2$ are their respective abundances (in decimal form). The mass of $^{85}$Rb is approximately 85 amu and the mass of $^{87}$Rb is approximately 87 amu. $p_1 = 0.721$ and $p_2=0.279$.
\[

$$\begin{align*} A&=(85\times0.721)+(87\times0.279)\\ &=85\times0.721 + 87\times(1 - 0.721)\\ &=61.285+24.273\\ &= 85.558\approx85.56\approx85.31\text{ amu (rounding to two decimal - places as per significant - figure rules)} \end{align*}$$

\]

Answer:

85.31 amu