Question

rubidium (rb) has two naturally occurring isotopes: rb - 85 and rb - 87. rb - 85 has a 72.20% abundance and a mass of 84.91 amu, and rb - 87 has a 27.80% abundance and a mass of 86.91 amu. part a calculate the atomic mass for rubidium using the weighted average mass method. express your answer using four significant figures and include the appropriate units. atomic mass of rb = value units

Explanation:

Step1: Convert percentages to decimals

$72.20\%=0.7220$, $27.80\% = 0.2780$

Step2: Calculate weighted - mass of Rb - 85

$m_1=0.7220\times84.91\ \text{amu}=61.30502\ \text{amu}$

Step3: Calculate weighted - mass of Rb - 87

$m_2=0.2780\times86.91\ \text{amu}=24.16098\ \text{amu}$

Step4: Calculate atomic mass of Rb

$m = m_1 + m_2=61.30502\ \text{amu}+24.16098\ \text{amu}=85.466\ \text{amu}\approx85.47\ \text{amu}$

Answer:

$85.47\ \text{amu}$