N₂O₅ is a molecular compound of two nonmetals, so prefixes give the atom counts: di- for two nitrogens and penta- for five oxygens. The final a of penta is dropped before oxide, giving pentoxide.
Question 2 True / False
CaCl₂ should be named calcium dichloride.
TTrue
FFalse
Answer: False
Calcium chloride is ionic. Calcium always forms Ca²⁺ and chloride is Cl⁻, so the formula follows from the charges and no prefix is needed.
Question 3 Multiple Choice
What is the name of H₂SO₃ dissolved in water?
ASulfuric acid
BHydrosulfuric acid
CSulfurous acid
DHyposulfurous acid
H₂SO₃ contains the sulfite ion, SO₃²⁻. An -ite anion gives an -ous acid, so the name is sulfurous acid. Sulfuric acid, H₂SO₄, comes from sulfate.
Question 4 Short Answer
Write the formula for dinitrogen tetroxide and say why it is named with prefixes.
Think about your answer, then reveal below.
Model answer: N₂O₄. Nitrogen and oxygen are both nonmetals, so the compound is molecular, and the prefixes di- and tetra- state the number of each atom.
Nonmetals can combine in several ratios (NO, NO₂, N₂O, N₂O₄, N₂O₅), so the name must state the ratio explicitly. Prefixes do that job.