MediumChemistryHigh School (11–12)The p-Block ElementsNumerical

Natural boron has two stable isotopes. Boron-10 has an atomic mass of 10.0129 u and boron-11 has…

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Natural boron has two stable isotopes. Boron-10 has an atomic mass of $10.0129\ \text{u}$ and boron-11 has $11.0093\ \text{u}$ . The average atomic mass of natural boron is $10.811\ \text{u}$ . Boron-10 absorbs neutrons strongly, and boron compounds are used in the control rods of nuclear reactors. Find the percentage of each isotope by number of atoms. Then find the mass of boron-10 in $250\ \text{g}$ of natural boron.

Need a reminder? The key ideaShow

To make a gaseous $3+$ ion, the three electrons leave one after the other. The energy is the sum $I_1 + I_2 + I_3$ . A weighted mean of isotope masses is $\bar{M} = x\,m_1 + (1 - x)\,m_2$ . Aluminium dissolves in alkali: $2\text{Al} + 2\text{NaOH} + 6\text{H}_{2}\text{O} \to 2\text{Na}[\text{Al}(\text{OH})_{4}] + 3\text{H}_{2}$ , so 2 mol of Al give 3 mol of H $_2$ .

The worked solution

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