Question

1. Find the number of moles that are in 2.93g of LiO2 and report it with the correct number of significant figures
a. 8.75 x 10^22 mol
b. 9.81 x 10^-2 mol
c. 0.0980589 mol
d. 87.5 mol

2. calculate the mass of Chromium (Cr) in magnesium dichromate (MgCr2 O7)
a. 21.6%
b. 23.1%
c. 43.3%

3. A gold wedding band contains 2.3 x 10^22 Au atoms. How many moles of gold are in the wedding band?
a. 3.8 x 10^-2 mol Au
b. 26.2 mol Au
c. 3.8 mol Au
d. 3. 92 mol Au

Answer 1

1.Mass of Li₂O = 2.93 g

Molecular weight of Li₂O = 29.88 g/mol

We know that

number of moles = mass/mol.wt.= 2.93 g/(29.88 g/mol) = 0.0980589 mol = 9.80589 × 10^–2 mol

Since 2.93 has 3 significant figures, the answer should be rounded to 3 sig.fig.

Number of moles = 9.81 × 10^–2mol

Option B is the right answer.

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2. Molecular weight of MgCr₂O₇ is

= (1* 24.31) + (2* 52.00) + (7*16.00) = 240.31 g/mol

Mass of Chromium in 1 mole of MgCr₂O₇ = 2*52.00 = 104 g

Mass percent of Chromium = mass of Cr/mass of MgCr₂O₇ = 104/240.31 * 100 = 43.27743 % = 43.3 %

Option C is the right answer.

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3. We know that

1 mole of any substance contains Avogadro’s number of atoms/molecules.

So, 1 mole of Au contains Avogadro’s number (6.022 × 10²³) of Au atoms

Number of moles of gold = 2.3 × 10²² Au atoms * 1 mol / 6.022 × 10²³ Au atoms = 0.0381933 mol

Number of moles of Gold = 0.0381933 mol = 3.82 × 10^–2mol

Option A is the right answer.