Practice Questions

Calculate the mass per cent of nitrogen, hydrogen, carbon, and oxygen in urea ($\text{CON}_2\text{H}_4$). (Atomic masses: C=12.01 u, O=16.00 u, N=14.01 u, H=1.008 u)

List the seven base physical quantities and their corresponding SI units.

Explain the difference between a homogeneous and a heterogeneous mixture with one example for each.

State the Law of Conservation of Mass.

Define an empirical formula.

The boiling point of ethanol is $78.37^\circ\text{C}$. Calculate this temperature in Kelvin.

Name the SI base unit for the amount of substance and its symbol.

Propose a simple, physical method to distinguish between a homogeneous mixture (like a sugar solution) and a pure liquid compound (like water) without using any specialized laboratory equipment. Justify your proposed method.

Analyze the number 0.052090 and determine the number of significant figures.

Define molarity.

Analyze the reaction for the combustion of propane: $\text{C}_3\text{H}_8(\text{g}) + 5\text{O}_2(\text{g}) \rightarrow 3\text{CO}_2(\text{g}) + 4\text{H}_2\text{O}(\text{l})$. Calculate the mass of water produced when $22$ g of propane is burnt completely. (Molar mass of $\text{C}_3\text{H}_8 = 44$ g/mol, $\text{H}_2\text{O} = 18$ g/mol)

Evaluate Dalton's postulate that "atoms are indivisible" in the context of both chemical and nuclear reactions. Justify its validity in each case.

Apply scientific notation to express the number $543,000,000$ with three significant figures.

A car's fuel efficiency is $32.0$ miles per gallon. Apply dimensional analysis to convert this to kilometers per liter. (Given: 1 mile = $1.609$ km, 1 gallon = $3.785$ L)

A solution is prepared by dissolving $5.85$ g of sodium chloride ($\text{NaCl}$) in enough water to make $200$ mL of solution. Calculate the molarity of the solution. (Molar mass of NaCl = $58.5$ g/mol)

You are tasked with preparing solutions of copper(II) sulfate for an experiment. (a) Design a step-by-step procedure to prepare $500.0 \text{ mL}$ of a $2.00 \text{ M}$ stock solution of $CuSO_4$ using solid copper(II) sulfate pentahydrate, $CuSO_4 \cdot 5H_2O$. Your design must include the necessary mass calculation. (Molar masses: Cu=63.5, S=32.1, O=16.0, H=1.0 g/mol). (b) From this stock solution, formulate a method, including calculations, to prepare $250.0 \text{ mL}$ of a $0.400 \text{ M}$ $CuSO_4$ solution. Justify your calculation using the principle of mole conservation.

Summarize three main postulates of Dalton's Atomic Theory.

Describe the three common scales used for measuring temperature and state the relationship between the Celsius and Kelvin scales.

Explain the difference between mass and weight. Identify the SI unit for mass.

Identify the number of significant figures in the number 0.05020.

An organic compound was found to contain $40.0\%$ carbon, $6.7\%$ hydrogen, and $53.3\%$ oxygen by mass. Analyze this data to determine its empirical formula.

Calculate the volume of $0.50$ M hydrochloric acid ($\text{HCl}$) solution required to react completely with $10.0$ g of calcium carbonate ($\text{CaCO}_3$) according to the equation: $\text{CaCO}_3(\text{s}) + 2\text{HCl}(\text{aq}) \rightarrow \text{CaCl}_2(\text{aq}) + \text{CO}_2(\text{g}) + \text{H}_2\text{O}(\text{l})$. (Molar mass of $\text{CaCO}_3 = 100.1$ g/mol)

Explain the terms 'precision' and 'accuracy' using an example.

Describe the classification of matter at the macroscopic level. Use a flowchart to illustrate the classification into pure substances and mixtures, and their further subdivisions. Provide an example for each category.

A compound containing only carbon, hydrogen, and oxygen is analyzed. Combustion of a $1.50$ g sample produces $2.20$ g of $\text{CO}_2$ and $0.90$ g of $\text{H}_2\text{O}$. The molar mass of the compound is determined to be $60.0$ g/mol. Examine the data to determine the empirical and molecular formulas of the compound.

Calculate the number of molecules in $0.25$ moles of ammonia ($\text{NH}_3$).

Calculate the mass in grams of a single atom of gold (Au). (Atomic mass of Au = $197$ u)

Justify why molality is often preferred over molarity as a unit of concentration for experiments that are conducted over a range of temperatures.

A student is tasked with determining the density of copper. The accepted literature value is $8.96 \text{ g/cm}^3$. The student records three measurements: $8.71 \text{ g/cm}^3$, $8.73 \text{ g/cm}^3$, and $8.72 \text{ g/cm}^3$. Evaluate these results in terms of precision and accuracy. Justify your conclusions with calculations.

An unknown organic compound is found to contain only Carbon, Hydrogen, and Oxygen. Analysis reveals its composition to be $40.0\%$ Carbon and $6.7\%$ Hydrogen by mass. Formulate a clear, step-by-step plan to determine its empirical formula from this data. Justify each step.

Design a laboratory experiment to verify the Law of Conservation of Mass using the precipitation reaction between aqueous solutions of lead(II) nitrate, $Pb(NO_3)_2$, and potassium iodide, $KI$. Your design must specify the apparatus, procedure, and the critical observation needed to validate the law.

Critique the following procedure for preparing a $0.5 \text{ M}$ solution of $NaOH$ in a $250 \text{ mL}$ volumetric flask: "Weigh approximately 5 g of $NaOH$ pellets on a watch glass, transfer them to the volumetric flask, and then add water up to the mark." Propose at least two critical improvements and justify their necessity.

Formulate a single unit factor (conversion factor) that can be used to convert a substance's density from units of $\text{g/cm}^3$ to $\text{kg/m}^3$. Justify the components of your factor.

Two compounds are formed between carbon and oxygen. Compound A contains $42.9\%$ carbon by mass, while Compound B contains $27.3\%$ carbon by mass. Formulate a proof to demonstrate that this data is consistent with the Law of Multiple Proportions.

The density of a $2.05$ M solution of acetic acid ($\text{CH}_3\text{COOH}$) in water is $1.02$ g/mL. Calculate the molality of the solution. (Molar mass of $\text{CH}_3\text{COOH} = 60.05$ g/mol)

Critique the statement: "An exact number, such as the '2' in the formula for water ($H_2O$), has an infinite number of significant figures." Justify whether this concept is always practically relevant in stoichiometric calculations.

Summarize the five basic laws of chemical combination that govern the formation of compounds from elements. State each law clearly.

Explain the five rules for determining the number of significant figures in a measurement. Provide a clear example for each rule.

Justify the international agreement in 1961 to use Carbon-12 as the standard for defining the atomic mass unit (amu). Propose one significant advantage and one potential disadvantage if the scientific community had decided to retain the lightest isotope, Hydrogen-1 ($^1H$), as the standard with a defined mass of exactly 1.

Critique the assertion that "Gay-Lussac's Law of Gaseous Volumes and Avogadro's Law are independent principles." Formulate a cohesive argument, using the reaction $H_2(g) + Cl_2(g) \rightarrow 2HCl(g)$ as an example, to demonstrate how Avogadro's Law provides the necessary theoretical foundation to explain Gay-Lussac's empirical observations.

In the synthesis of ammonia, $280$ g of dinitrogen gas ($\text{N}_2$) is mixed with $90$ g of dihydrogen gas ($\text{H}_2$). The reaction is: $\text{N}_2(\text{g}) + 3\text{H}_2(\text{g}) \rightarrow 2\text{NH}_3(\text{g})$. (a) Identify the limiting reagent. (b) Calculate the maximum mass of ammonia that can be produced. (c) Calculate the mass of the excess reagent that remains unreacted.

Define the terms atomic mass, average atomic mass, molecular mass, and formula mass. Explain how each is determined and provide an example for each.

Boron has two stable isotopes, $^{10}\text{B}$ and $^{11}\text{B}$. Their respective masses are $10.0129$ u and $11.0093$ u, and the average atomic mass of boron is $10.811$ u. Calculate the natural abundance of each isotope.

Design a complete experimental strategy to determine the molecular formula of a newly synthesized, unknown volatile liquid that is known to contain only carbon, hydrogen, and oxygen. Your strategy must outline three distinct experimental stages and justify the choice of method for each of the first two stages.

In an industrial reactor for the Haber process, $N_2(g) + 3H_2(g) \rightleftharpoons 2NH_3(g)$, a batch is prepared with $100.0 \text{ kg}$ of dinitrogen ($N_2$) and $25.0 \text{ kg}$ of dihydrogen ($H_2$). (a) Justify, with calculations, which reactant is the limiting reagent. (b) Propose a calculation to determine the maximum theoretical yield of ammonia ($NH_3$) in kilograms. (c) Evaluate the economic reasoning behind using one reactant in excess in a large-scale industrial process.