A sealed flask contains 90 g of iron filings and 60 g of sulfur, with no other material included in the mass calculation. They are allowed to react without matter entering or leaving. What is the combined mass of the products and any unreacted material afterward?
(a)300 g
(b)150 g
(c)90 g
(d)60 g
Show answer
(b) 150 g
The combined mass of iron and sulfur is 90 g + 60 g = 150 g. In a sealed flask, no matter enters or leaves, so the combined mass of the material remains 150 g after the reaction. This includes any unreacted material; the given amounts need not both be completely consumed.
Question 2
Why does pure water always have the same hydrogen-to-oxygen mass ratio?
(a)A pure compound can have any element ratio
(b)Its elements combine in a fixed proportion in that compound.
(c)Mass disappears during a chemical reaction
(d)Chemical reactions create new atoms
Show answer
(b) Its elements combine in a fixed proportion in that compound.
Constant composition distinguishes a pure compound from a variable mixture.
Properties and masses of ionic and covalent compounds
Question 3
Why does molten sodium chloride conduct electricity while solid sodium chloride does not?
(a)Melting breaks the electron pairs shared between sodium and chlorine.
(b)Melting only speeds up ions that already move about inside the solid.
(c)Melting turns the ions into free electrons that carry the current.
(d)Its ions can move when molten but are fixed in the solid lattice.
Show answer
(d) Its ions can move when molten but are fixed in the solid lattice.
A current needs charged particles that are free to move. Sodium chloride is made of sodium ions and chloride ions. In the solid these ions are held in fixed places in the lattice, so no charge flows. Melting breaks the lattice apart, the ions move, and the melt conducts.
Question 4
Using H=1 and O=16, what is the molecular mass of H₂O?
(a)Mass calculation ignores subscripts
(b)18 u
(c)Covalent compounds always have high melting points
(d)A solid ionic compound conducts through moving ions
Show answer
(b) 18 u
The two hydrogen atoms contribute 2 u and oxygen contributes 16 u.
Covalent and ionic bonding
Question 5
How do two chlorine atoms form Cl₂?
(a)Transferring one electron in a covalent bond
(b)Sharing one electron pair in a covalent bond
(c)Sharing two electron pairs in a covalent bond
(d)Sharing one electron pair in an ionic bond
Show answer
(b) Sharing one electron pair in a covalent bond
Each chlorine atom has seven valence electrons and contributes one electron to a shared pair. This pair completes both octets and forms a single covalent bond.
Question 6
How does sodium chloride form?
(a)Electron transfer from sodium to chlorine, forming an ionic compound with an overall positive charge
(b)Electron transfer from sodium to chlorine, forming a covalent compound
(c)Electron transfer from sodium to chlorine, forming an ionic compound with no overall charge
(d)Sharing an electron pair between sodium and chlorine, forming an ionic compound
Show answer
(c) Electron transfer from sodium to chlorine, forming an ionic compound with no overall charge
Sodium transfers one electron to chlorine, forming Na⁺ and Cl⁻. Electrostatic attraction between these ions forms ionic bonds. Their equal and opposite charges give sodium chloride no overall charge.
Writing chemical formulae
Question 7
What is the simplest whole-number ratio of Mg²⁺ ions to Cl⁻ ions in magnesium chloride?
(a)2 : 1
(b)1 : 2
(c)2 : 4
(d)1 : 1
Show answer
(b) 1 : 2
One Mg²⁺ ion carries a charge of +2 and one Cl⁻ ion carries −1. Two chloride ions are needed to balance one magnesium ion, so the total charge is zero. The simplest whole-number ratio is 1 : 2.
Question 8
Aluminium oxide contains Al³⁺ and O²⁻ ions. Its formula is written as Al₄O₆. What is its formula in the simplest whole-number ratio?
(a)Al₂O₃
(b)Al₄O₆
(c)AlO
(d)Al₃O₂
Show answer
(a) Al₂O₃
Divide both subscripts in Al₄O₆ by 2 to get Al₂O₃. Two Al³⁺ ions give +6 and three O²⁻ ions give −6, so the total charge is zero.
Common misconception in Atomic Foundations of Matter
Both sides contain four H atoms and two O atoms. The product stays water.
2H₂ + O₂ → 2H₂O gives four H atoms and two O atoms on each side, with water as the product.
Cookies
We use cookies, including Meta’s and, if a ChatGPT advert brought
you, OpenAI’s, to see how visitors find us.
They are never used on your child’s screens in the app. You can turn them
off here, and change your mind any time.
Privacy policy
