SK015 Pre-Lab 1 Determination Formula Unit of A Compound

Which definition best to describe a compound?

• A mixture of two or more elements physically combined

• A pure substance composed of two or more elements chemically combined in a fixed ratio

• A group of atoms bonded by metallic bonds

• An element with multiple isotopes

What does the term formula unit refers to?

• A single molecule in a gas

• The smallest ratio of elements (or ion) in an ionic compound

• A set of covalently bonded atoms

• A mole of a compound

Which of the following best describes an empirical formula?

• Shows actual number of atoms in a molecule

• Shows the simplest whole-number ratio of atoms in a compound

• Only applies to gases

• Refers to the mass ratio of elements

The molecular formula (MF) of a compound is always:

• Less than its empirical formula

• Equal to or a multiple of the empirical formula

• Based on melting point

• Irrelevant for ionic compounds

A compound contains 0.20 mol of Al and 0.30 mol of O. What is its empirical formula?

[Ar Al=27, O=16]

• AlO

• Al₂O₃

• AlO₃

• Al₃O₂

Which of the following is a safety steps in this experiment?

• Always add water to acid

• Heat the compound until it melts

• Handle hot equipment with tongs

• Use broken glassware if cleaned

Why must the apparatus be dried completely before weighing?

• To remove oxygen

• To prevent water from affecting the mass

• To dissolve the compound

• To remove excess acid

Why should the solid compound not be heated to its melting point (or heated until melt)?

• It becomes more reactive

• It may vaporize or decompose and altering the mass

• It bonds with air molecules

• The experiment requires a liquid phase

Why is the solid product obtained must be reheated, cooled, and reweighed?

• To ensure complete combustion

• To allow time for settling

• To ensure consistent mass for accurate measurement

• To check if melting occurred

After heating is done, why is it important to cool the crucible before weighing solid product?

• To avoid errors from convection currents of hot apparatus

• To allow hydrogen to escape freely

• To remove water vapor completely

• To prevent zinc from melting