Quiz 3 - Periodic Table & Atomic Mass | The Periodic Table and Atomic Mass

Let x be the fraction of 185Re, then (1 - x) is the fraction of 187Re. The average atomic mass equation is:

185x + 187 (1-x) = 186.2
185x + 187 − 187x = 186.2
− 2x +1 87 = 186.2
− 2x = − 0.8
x = 0.4

Thus, the mole percent of ¹⁸⁵Re is 40%.

Pharmacolite's formula is CaHAsO₄•2H₂O ⇒ It has 5 hydrogen atoms per formula unit.

Moles of pharmacolite = $\frac{2.50 \mathrm{g}}{216.0 \mathrm{g}.{\mathrm{mol}}^{-1}}$ = 0.0116 mol

Number of hydrogen atoms = 5 x 0.0116 mol × 6.022 × 10²³ atoms/mol = 3.48 × 10²² atoms.

Among the given metals, Cesium (Cs) reacts most vigorously with water due to its position in Group 1 of the periodic table. Alkali metals like Cs react violently with water to form hydroxides and hydrogen gas.

1. The balanced equation is: 2 Li + 2 H₂O → 2 LiOH + H₂

2. Molar mass of LiOH = 23.95 g/mol.

3. Moles of LiOH produced from 12 g:  $\frac{12 \mathrm{g}}{23.95 \mathrm{g}.{\mathrm{mol}}^{-1}}$ ≈ 0.501 mol

4. Moles of Li required (1:1 ratio): 0.501 mol

5. Mass of Li required: 0.501 mol × 6.94 g/mol ≈ 3.48 g.

Bromine (Br) is the only element among the options that is a liquid at room temperature (25 ºC) and 1 atm pressure. Fluorine and chlorine are gases, while iodine is a solid under these conditions.

• Calculate the moles of FeS₂ in 3.00 g:
  Moles of FeS₂ =  $\frac{3.00 \mathrm{g}}{120.0 \mathrm{g}.{\mathrm{mol}}^{-1}}$ = 0.025 mol

• Each mole of FeS₂ contains 2 moles of sulfur atoms:
  Moles of S atoms = 0.025 mol × 2 = 0.050 mol

• Convert moles of sulfur atoms to the number of atoms using Avogadro's number (6.022 x 10²³ atoms/mol):
  Number of S atoms = 0.050 mol x 6.022 x 10²³ atoms/mol = 3.011 x 10²² atoms

White phosphorus is stored under water to prevent it from reacting with air, as it is highly reactive and can ignite spontaneously in contact with oxygen.

• Calculate the moles of CuSO₄•5H₂O in 0.10 g:
  Moles of CuSO₄•5H₂O = $\frac{0.10 \mathrm{g}}{249.7 \mathrm{g}.{\mathrm{mol}}^{-1}}$  ≈ 4.0 x 10⁻⁴ mol

• Each mole of CuSO₄•5H₂O contains 5 moles of water molecules (H₂O).

• Calculate the moles of water in the sample:
  Moles of H₂O= 5 x 4.0 x 10⁻⁴ mol = 2.0 x 10⁻³ mol

• Convert moles of water to the number of molecules using Avogadro's number (6.022 x 10²³ molecules/mol):
  Number of H₂O molecules = 2.0 x 10⁻³ mol x 6.022 x 10²³ molecules/mol = 1.2 x 10²¹ molecules

Mercury's high density allows for a shorter column to measure atmospheric pressure. A barometer using water would need to be approximately 13.6 times taller than one using mercury to measure the same pressure.

• The molar mass of water (H₂O) is 18 g/mol.
• One mole of water contains Avogadro's number of molecules (6.022 x 10²³ molecules).
• Calculate the mass of one molecule of water:
  Mass of one molecule = $\frac{18 \mathrm{g}.{\mathrm{mol}}^{-1}}{6.022 \mathrm{x} {10}^{23} \mathrm{molecules}.{\mathrm{mol}}^{-1}}$ ≈ 2.99 x 10⁻²³ g