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

1

Rhenium (Re) consists of two stable isotopes, 185Re and 187Re. The average atomic mass of Re is 186.2 amu. What is the mole percent of 185Re in rhenium?

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%.

2

How many hydrogen atoms are in 2.50 g of pharmacolite, CaHAsO4•2H2O (M = 216.0)?

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

Moles of pharmacolite =  = 0.0116 mol

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

3

Which metal reacts most vigorously with water?

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.

4

Lithium reacts with water to form lithium hydroxide. What mass of lithium is required to produce 12 g of lithium hydroxide?  (MLi = 6.94 g.mol-1

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:   ≈ 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.

5

Which element is a liquid at 25 ºC and 1 atm pressure?

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.

6

How many sulfur atoms are in 3.00 g of iron pyrite, FeS2 (M = 120.0)?

• Calculate the moles of FeS2 in 3.00 g:
Moles of FeS2 =   = 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

7

Which substance is stored in contact with water to prevent it from reacting with air?

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.

8

How many water molecules are in a 0.10 g sample of CuSO4•5H2O (MM = 249.7)?

• Calculate the moles of CuSO4•5H2O in 0.10 g:
Moles of CuSO4•5H2O =   ≈ 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

9

What is the major reason for using mercury (rather than water) in barometers?

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.

10

What is the mass of one molecule of water in grams?

• 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 =  ≈ 2.99 x 10−23 g