Chemistry Concepts Assessment

What is the percentage composition of water (H2O)?

• H: 15%, O: 85%

• H: 5%, O: 95%

• H: 11.11%, O: 88.89%

• H: 20%, O: 80%

Answer explanation

To find the percentage composition of water (H2O), calculate the molar mass: H (1g/mol) x 2 = 2g/mol, O (16g/mol) = 16g/mol. Total = 18g/mol. H: (2/18) x 100 = 11.11%, O: (16/18) x 100 = 88.89%. Thus, H: 11.11%, O: 88.89% is correct.

Calculate the percentage composition of carbon in CO2.

• 25.00%

• 30.50%

• 20.15%

• 27.29%

Answer explanation

To find the percentage composition of carbon in CO2, calculate the molar mass: C = 12.01 g/mol, O = 16.00 g/mol (2 O = 32.00 g/mol). Total = 44.01 g/mol. Carbon's percentage = (12.01/44.01) * 100 = 27.29%. Thus, the correct answer is 27.29%.

How do you determine the empirical formula from percent composition?

• The empirical formula is determined by the color of the elements.

• The empirical formula is calculated using the boiling point of the compound.

• The empirical formula is derived from the mole ratio of the elements in the compound.

• The empirical formula is based on the molecular weight of the compound.

Answer explanation

The empirical formula is derived from the mole ratio of the elements in the compound, which is calculated from the percent composition. This method allows us to determine the simplest whole-number ratio of atoms.

Given 40% carbon and 60% hydrogen, what is the empirical formula?

• C4H10

• C3H6

• C2H5

• CH18

Answer explanation

To find the empirical formula, convert the percentages to moles: 40g C (3.33 mol) and 60g H (60 mol). The ratio is 1:18, simplifying to CH18, which is the correct empirical formula.

What is the empirical formula of a compound with 3 moles of carbon and 8 moles of hydrogen?

• C3H8

• C4H10

• C2H6

• C3H6

Answer explanation

The empirical formula is determined by the simplest whole number ratio of moles of each element. With 3 moles of carbon and 8 moles of hydrogen, the formula is C3H8, which represents propane.

How do you find the molecular formula from the empirical formula?

• Molecular formula = Empirical formula + Molar mass of compound

• Molecular formula = Empirical formula / Molar mass of empirical formula

• Molecular formula = (Empirical formula subscripts) × (Molar mass of compound / Molar mass of empirical formula)

• Molecular formula = (Empirical formula subscripts) + (Molar mass of compound)

Answer explanation

To find the molecular formula from the empirical formula, multiply the subscripts of the empirical formula by the ratio of the molar mass of the compound to the molar mass of the empirical formula. This gives the correct molecular formula.

If the empirical formula is CH2 and the molar mass is 28 g/mol, what is the molecular formula?

• C4H8

• C2H4

• CH3

• C3H6

Answer explanation

To find the molecular formula, divide the molar mass (28 g/mol) by the empirical formula mass (14 g/mol for CH2). This gives 2, so multiply the subscripts in CH2 by 2, resulting in C2H4.

What is the molar mass of NaCl?

• 58.44 g/mol

• 58.00 g/mol

• 60.00 g/mol

• 55.45 g/mol

Answer explanation

The molar mass of NaCl is calculated by adding the atomic masses of sodium (Na, 22.99 g/mol) and chlorine (Cl, 35.45 g/mol), resulting in 58.44 g/mol, making it the correct choice.

Calculate the molar mass of C6H12O6.

• 165.45 g/mol

• 200.25 g/mol

• 150.12 g/mol

• 180.18 g/mol

Answer explanation

To calculate the molar mass of C6H12O6, sum the atomic masses: C (6x12.01) + H (12x1.01) + O (6x16.00) = 180.18 g/mol. Thus, the correct answer is 180.18 g/mol.

How do you convert grams of a substance to moles?

• moles = grams - molar mass

• moles = grams / molar mass

• moles = grams * molar mass

• moles = grams + molar mass

Answer explanation

To convert grams to moles, use the formula: moles = grams / molar mass. This formula allows you to find the number of moles by dividing the mass of the substance by its molar mass, making the second option the correct choice.