UAE TLS Chemistry Test common mistakes
UAE TLS Chemistry Test · Understand the traps and how to avoid them.
When balancing an equation, changing the subscript in a formula (e.g., writing H2O2 instead of H2O) to make atom counts match rather than placing a coefficient in front of the formula.
Students treat the equation as an arithmetic puzzle and reach for the fastest way to equalize both sides, forgetting that subscripts define the identity of the substance and cannot be altered without changing the chemistry.
What to do instead
Enforce a two-column habit: write the unbalanced equation, then adjust only the numbers written to the left of each formula; before finalizing, re-copy every formula unchanged from the original and verify each element's count on both sides.
Reading '0.50 mol of Cl2' as 0.50 mol of chlorine atoms and using 0.50 in the mole ratio, instead of treating the diatomic molecule as containing 1.00 mol of Cl atoms.
The formula is read as a label rather than as a count of atoms, so the distinction between moles of molecules and moles of atoms collapses.
What to do instead
Require an explicit conversion line: moles of molecules × (atoms per molecule) = moles of atoms, written out before substituting into any ratio.
Using the limiting-reactant quantity to compute product mass without first comparing the mole ratio of each reactant to the balanced equation.
Students assume the reactant given first, or the one with the smaller mass, is limiting, and skip the comparison step entirely.
What to do instead
Standardize a three-step check: convert every reactant to moles, divide each by its coefficient, and identify the smallest quotient as limiting before any product calculation.
Drawing a Lewis structure that satisfies the octet on carbon but leaves a halogen or oxygen with too few electrons, or ignoring formal charge when several valid skeletons exist.
Students count only the central atom's electrons and stop as soon as it looks complete, rather than tracking total valence electrons against electrons placed.
What to do instead
Use a fixed electron ledger: total valence electrons, subtract bonding pairs, distribute the remainder as lone pairs, then compute formal charges and prefer the structure with charges closest to zero.
Predicting molecular geometry from the number of bonded atoms alone, so that a molecule with lone pairs on the central atom is assigned the electron-pair geometry instead of the molecular shape.
VSEPR is memorized as 'count the atoms' rather than 'count electron domains, then subtract lone pairs', and lone pairs are visually easy to overlook.
What to do instead
Always count electron domains first, assign the domain geometry, then remove lone-pair positions to get the molecular shape, and state the bond angle consequence explicitly.
Writing the equilibrium constant expression with products over reactants but omitting the exponents, or including a pure solid or liquid in the expression.
The expression is recalled as a general 'products over reactants' pattern without the stoichiometric powers, and heterogeneous equilibria are treated like homogeneous ones.
What to do instead
Derive Kc from the balanced equation each time, raising each concentration to its coefficient and crossing out any species in the solid or liquid state before writing the final expression.
Applying Le Chatelier's principle by shifting the equilibrium in the direction that 'relieves' a change without verifying whether the change actually affects the reaction quotient, e.g., predicting a shift when an inert gas is added at constant volume.
The principle is applied as a slogan rather than through Q versus K reasoning, so any disturbance is assumed to cause a shift.
What to do instead
For every disturbance, write Q with the new conditions and compare it to K; if Q is unchanged, state explicitly that there is no shift and explain why.
Using the full concentration of a weak acid as the hydrogen ion concentration when computing pH, instead of solving the equilibrium with Ka.
The strong-acid habit of [H+] equals the acid concentration is carried over to weak acids, and the small extent of ionization is ignored.
What to do instead
Set up an ICE table with the Ka expression, apply the small-x approximation only after checking that Ka is small relative to concentration, and verify the approximation or solve the quadratic if it fails.
Assigning oxidation numbers by counting all electrons in a bond to the more electronegative atom even when the bond is between identical atoms, or by treating oxygen as always -2 in peroxides.
Rules are applied as absolutes without checking the exceptions and without distinguishing homonuclear from heteronuclear bonds.
What to do instead
Work through each atom with the rule hierarchy: elements in their standard state are zero, then group rules, then exceptions such as peroxides and hydrides, and check that the sum equals the overall charge.
Naming an organic compound by listing substituents alphabetically without numbering the parent chain to give the lowest locants, or choosing a parent chain that does not contain the principal functional group.
Students prioritize the substituent list over the IUPAC selection rules, and the longest chain is chosen by eye rather than by the priority of the functional group.
What to do instead
Apply the rules in a fixed order: identify the principal functional group, select the parent chain containing it, number to minimize its locant, then name substituents alphabetically with locants.