MTTC Chemistry (018) common mistakes
MTTC Chemistry (018) · Understand the traps and how to avoid them.
When reporting a result from a multi-step calculation, the candidate counts decimal places instead of significant figures and rounds at every intermediate step, then reports a final value with more digits than the least precise measurement justifies.
The candidate memorized surface rules such as 'count the digits after the decimal' rather than understanding measured uncertainty, exact numbers, and when rounding should occur.
What to do instead
Practice labeling every quantity as measured or exact, keep at least one guard digit through intermediate steps, apply multiplication/division and addition/subtraction rules separately, and round only at the end.
Writes the electron configuration of chromium as [Ar]4s2 3d4 and copper as [Ar]4s2 3d9, and when forming transition-metal cations removes 3d electrons before 4s electrons.
The candidate relies on a simplified Aufbau diagram and the phrase '4s fills before 3d' without accounting for half-filled and filled subshell stability or the fact that 4s is higher in energy in positive ions.
What to do instead
Draw orbital box diagrams for the first-row transition metals, memorize the Cr and Cu exceptions, and practice cation configurations by removing 4s electrons first.
Predicts molecular geometry from the number of bonded atoms only, so H2O is called linear and SO2 is called linear; the candidate does not subtract lone pairs from the electron-domain count before assigning shape.
VSEPR is reduced to 'count the bonds' rather than 'count electron domains and then account for lone-pair repulsion.'
What to do instead
Use the AXE method: count electron domains around the central atom, assign the electron-domain geometry, then remove lone pairs to obtain the molecular geometry; practice with H2O, SO2, NH3, and SF4.
Chooses the limiting reactant by comparing starting masses or volumes rather than moles, then uses that reactant's mass directly in a mole ratio to find product yield.
The candidate uses a 'more grams means excess' heuristic and skips the required gram-to-mole conversion before comparing stoichiometric amounts.
What to do instead
Always convert every reactant to moles, divide each by its coefficient in the balanced equation, compare the resulting quotients, and use the smallest quotient as the limiting amount.
States that adding a catalyst increases the equilibrium constant or the equilibrium yield of product.
The candidate conflates reaction rate with thermodynamic equilibrium and does not distinguish kinetic effects from changes in the equilibrium position.
What to do instead
Review that a catalyst lowers activation energy for both forward and reverse reactions, speeds attainment of equilibrium, and leaves K, ΔH, and equilibrium concentrations unchanged.
Writes an equilibrium-constant expression that includes pure solids or liquids, for example including CaCO3(s) in Kc = [CaO][CO2]/[CaCO3] for the decomposition of calcium carbonate.
The candidate treats every substance in the equation as a concentration term and has not learned that pure solids and liquids have constant activity.
What to do instead
Omit pure solids and pure liquids from K expressions, set their activities to 1, and practice heterogeneous equilibria involving carbonates, water, and sparingly soluble salts.
Calculates the pH of 0.10 M acetic acid as -log(0.10) = 1.00, without using Ka or an ICE table.
The strong-acid formula [H+] = initial acid concentration is overgeneralized to weak acids, and the candidate does not check whether dissociation is complete.
What to do instead
For weak acids, write the Ka expression, set up an ICE table, solve for [H+] with the square-root approximation, and verify that the approximation is valid; compare with a strong acid of the same concentration.
Uses molarity instead of molality in colligative-property calculations and sets the van't Hoff factor i = 1 for electrolytes such as NaCl or CaCl2.
Molarity is more familiar from earlier solution calculations, and the candidate forgets that colligative properties depend on the number of dissolved particles.
What to do instead
Use molality for boiling-point elevation and freezing-point depression, assign i based on the number of ions per formula unit for strong electrolytes, and practice with nonelectrolytes, NaCl, and CaCl2.
In an experiment involving several variables, the candidate changes temperature and reactant concentration at the same time, then attributes the observed rate change to one factor.
The candidate does not identify independent, dependent, and controlled variables and treats a confounded design as if it were a controlled experiment.
What to do instead
Before collecting data, write out the independent variable, dependent variable, and all controlled variables; design trials that change one factor at a time and include a control group.
Classifies an aldehyde as a ketone because both contain a C=O group, and predicts that oxidation of a tertiary alcohol produces an aldehyde.
The candidate memorized 'carbonyl means ketone' without noting the carbonyl position or the degree of substitution of the alcohol carbon.
What to do instead
Use structure-based rules: aldehydes have the carbonyl at the end of a chain and react with Tollens' reagent; primary alcohols oxidize to aldehydes then carboxylic acids, secondary alcohols to ketones, and tertiary alcohols do not oxidize under mild conditions.