Formulae & Relative Masses (Cambridge (CIE) IGCSE Co-ordinated Sciences (Double Award): Chemistry): Flashcards
Exam code: 0654 & 0973
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What information does a molecular formula give?
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What information does a molecular formula give?
A molecular formula shows the types of atoms in a molecule (by their chemical symbols) and the number of each type (shown by subscript numbers).
For example, H2O contains 2 hydrogen atoms and 1 oxygen atom.
True or False?
All seven diatomic elements exist as molecules made of two identical atoms.
True.
The seven diatomic elements (H2, N2, O2, F2, Cl2, Br2 and I2) all exist as molecules containing two atoms of the same element bonded together.
The seven diatomic elements are H2, ______, O2, ______, Cl2, Br2 and I2.
The seven diatomic elements areH2, N2, O2, F2, Cl2, Br2 and I2.
How do you use the swap-and-drop method to deduce the formula of an ionic compound?
Write the charges of both ions. Swap the numbers (ignoring the sign) and use them as subscripts for the other ion.
For example, Ca2+ and Cl- gives CaCl2, because the 2 from Ca2+ becomes the subscript for Cl, and the 1 from Cl- is not written.
True or False?
The formula of copper(II) chloride is CuCl.
False.
Copper(II) ions have a 2+ charge (Cu2+) and chloride ions have a 1- charge (Cl-). Two chloride ions are needed to balance the charge, giving the formula CuCl2.
In the formula Mg(NO3)2, there are ______ nitrogen atoms, ______ oxygen atoms and ______ magnesium atom.
In the formula Mg(NO3)2, there are 2 nitrogen atoms, 6 oxygen atoms and 1 magnesium atom.
Give the formula of the ionic compound formed between iron(III) ions (Fe3+) and sulfate ions (SO42-).
The formula is Fe2(SO4)3.
Two Fe3+ ions (total 6+) are balanced by three SO42- ions (total 6-), giving no overall charge.
What is a word equation?
A word equation represents a chemical reaction using the full names of the reactants and products. Reactants are written on the left of the arrow and products on the right. The arrow means "produces" or "to form".
True or False?
When balancing a symbol equation, you can change the subscript numbers in a formula to make the atoms balance.
False.
Changing the subscripts would change the substance itself. To balance an equation, only the coefficients (numbers in front of each formula) can be changed.
State symbols show the physical state of each substance: (s) for ______, (l) for ______, (g) for gas and (aq) for ______.
State symbols show the physical state of each substance: (s) for solid, (l) for liquid, (g) for gas and (aq) for aqueous (dissolved in water).
What is an ionic equation and how is it constructed?
An ionic equation shows only the ions that take part in the reaction.
To write an ionic equation:
Write the full balanced symbol equation
Replace soluble ionic compounds with their component ions
Cancel any ions that appear unchanged on both sides (spectator ions).
True or False?
In a balanced symbol equation, the total mass of reactants equals the total mass of products.
True.
This follows from the law of conservation of mass. Atoms are rearranged in a chemical reaction but cannot be created or destroyed, so the total mass is the same on both sides.
Balance the following equation:
__Al (s) + __CuO (s) → __Al2O3 (s) + __Cu (s)
The balanced equation is:
2Al (s) + 3CuO (s) → Al2O3 (s) + 3Cu (s).
Balance Al first (2 on right → 2 on left), then O (3 in Al2O3 → 3 CuO on left), then Cu (3 CuO → 3 Cu on right).
In a balanced symbol equation, the ______ of each element must be equal on both sides, because atoms cannot be ______ or destroyed in a chemical reaction.
In a balanced symbol equation, the number of each element must be equal on both sides, because atoms cannot be created or destroyed in a chemical reaction.
What is the relative atomic mass (Ar) of an element?
The relative atomic mass (Ar) is the mass of one atom of an element compared to one-twelfth the mass of a carbon-12 atom. It has no units and can be found in the Periodic Table.
True or False?
The relative atomic mass of an element is always the same as its proton number.
False.
The relative atomic mass is always larger than (or equal to, for hydrogen) the proton number, because it accounts for the mass of both protons and neutrons in the nucleus.
The relative molecular mass (Mr) of a compound is calculated by ______ the relative atomic masses of all the ______ in the formula.
The relative molecular mass (Mr) of a compound is calculated by adding the relative atomic masses of all the atoms in the formula.
Calculate the Mr of calcium hydroxide, Ca(OH)2. (Ar: Ca = 40, O = 16, H = 1)
Mr = 40 + (2 × 16) + (2 × 1) = 40 + 32 + 2 = 74.
The subscript 2 applies to the whole (OH) group, giving 2 oxygen atoms and 2 hydrogen atoms.
True or False?
The relative atomic mass of an element has units of grams.
False.
Relative atomic mass is a ratio and has no units. It compares the mass of one atom to one-twelfth the mass of a carbon-12 atom.
Calculate the Mr of ammonium sulfate, (NH4)2SO4. (Ar: N = 14, H = 1, S = 32, O = 16)
Mr = (2 × 14) + (8 × 1) + 32 + (4 × 16) = 28 + 8 + 32 + 64 = 132.
The subscript 2 applies to the whole (NH4) group, giving 2 nitrogen atoms and 4 x 2 = 8 hydrogen atoms.
Carbon-12 is used as the ______ for relative atomic masses. The Ar of magnesium is ______, meaning magnesium is ______ times heavier than carbon.
Carbon-12 is used as the standard for relative atomic masses. The Ar of magnesium is 24, meaning magnesium is twice as heavy as carbon.
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