3.1 Formulae (Cambridge (CIE) O Level Chemistry): Flashcards
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Define chemical formula.
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Cards in this collection (18)
Define chemical formula.
A chemical formula shows the types of atoms present in a compound and the ratio in which they are combined. For example, H2O shows that water contains 2 hydrogen atoms for every 1 oxygen atom.
Element symbols always have the ______ letter capitalised. For a two-letter symbol, the second letter is always ______.
Element symbols always have the first letter capitalised. For a two-letter symbol, the second letter is always lowercase.
True or False?
The formula of magnesium nitride can be written as Mg3+3N3-2.
False.
The formula of magnesium nitride is Mg3N2. Ion charges are used to work out the formula but are not included in the final formula — only the symbols and subscripts remain.
To deduce the formula of aluminium oxide, aluminium has a combining power of ______ and oxygen has a combining power of ______. Cross-multiplying gives the formula ______.
Aluminium has a combining power of 3 and oxygen has a combining power of 2. Cross-multiplying gives the formula Al2O3.
What is the difference between the molecular formula and the empirical formula of a compound?
The molecular formula gives the actual number of atoms of each element in one molecule.
The empirical formula gives the simplest whole number ratio.
For example, ethanoic acid has the molecular formula C2H4O2 but the empirical formula CH2O.
True or False?
If a question asks for the formula of a substance, giving the correct name instead will earn the mark.
False.
If a question asks for a formula, only a formula is acceptable.
If a question asks for a name, a formula is only accepted if it is perfectly correct. Always read the question carefully.
Define empirical formula.
The empirical formula is the simplest whole number ratio of the atoms of each element present in a compound. The formula of an ionic compound is always an empirical formula.
Group II elements form ions with a ______ charge, and Group VII elements form ions with a ______ charge.
Group II elements form ions with a 2+ charge, and Group VII elements form ions with a 1- charge.
Deduce the formula of magnesium nitrate. (Mg2+; NO3-)
Two nitrate ions are needed to balance one Mg2+ ion, giving the formula Mg(NO3)2. Brackets are placed around a compound ion when more than one is needed in the formula.
True or False?
The formula of calcium chloride can be written as Ca2+Cl-2.
False.
Ion charges are used to determine the formula but must not appear in the final answer. The correct formula is CaCl2.
The formula of ammonia is ______, whereas the formula of the ammonium ion is ______.
The formula of ammonia is NH3, whereas the formula of the ammonium ion is NH4+.
True or False?
A phosphide ion and a phosphate ion are the same thing — both are negatively charged ions of phosphorus.
False.
A phosphide ion (P3-) is a single negatively charged phosphorus ion. A phosphate ion (PO43-) is a compound ion containing phosphorus and oxygen. They are completely different species.
What do state symbols show in a chemical equation?
State symbols show the physical state of each substance in a chemical equation: (s) = solid, (l) = liquid, (g) = gas, (aq) = dissolved in water (aqueous solution).
When balancing a chemical equation, you may only change the ______ in front of each formula. You must never change the ______ of any substance.
You may only change the coefficients in front of each formula. You must never change the formula of any substance.
True or False?
Chlorine must be written as Cl2 in a balanced symbol equation, not as Cl.
True.
Chlorine is a diatomic molecule and must be written as Cl2. The seven diatomic non-metal elements are H2, N2, O2, F2, Cl2, Br2 and I2.
What is a spectator ion, and how is it treated in an ionic equation?
A spectator ion is an ion that appears unchanged on both sides of an equation.
Spectator ions are removed when writing the ionic equation, leaving only the ions that actually participate in the reaction.
In a chemical equation, a substance dissolved in water is given the state symbol ______, while a solid precipitate formed in solution is given the state symbol ______.
A substance dissolved in water is given the state symbol (aq), while a solid precipitate is given the state symbol (s).
True or False?
To balance the equation Al + Cl2 → AlCl3, you can change AlCl3 to AlCl2 to make balancing easier.
False.
You must never change a chemical formula when balancing an equation. Only the coefficients may be changed. The correctly balanced equation is:
2Al + 3Cl2 → 2AlCl3.
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