Cooking with moles in chemistry

academics chemistry High School

Anyone who has had some contact with chemistry knows how important the mole is to chemistry. 

So what’s this mole got to do with chemistry anyway?

A mole is just a unit of measure; one mole refers to 6.02*1023 things. If I say that I have a mole of chairs, it means I have 6.02*1023 chairs.  A mole of molecules refers to 6.02*1023 molecules, two moles to 12.04*1023 molecules and so on.  

In chemistry, the mole is used to avoid working with inconvenient numbers. For instance, it is annoying to say 6.02*1023 molecules and much easier to say 1 mole. By the way, 6.02*1023 is also known as Avogradro’s number.

The mole is so important to chemistry because chemists no longer work with large/ inconvenient numbers, but instead use the mole as a unit.  The mole bridges all the different quantities you will come across in chemistry problems. For example, in a chemical reaction, the coefficients tell you about the moles of that particular molecule.

How to deal with mole calculations in chemical reactions?

Let’s say you have to balance the following equation:

C3H8    +  O2-------> CO2   +   H2O

To do this, you have to make sure you have the same number of atoms in both sides of the equation, for each of the atoms present in the equation. So, you have 3 moles of C on the left side, but just one on the right side. To increase the number of C on the right, you can just add a number 3 in front of CO2. When you add a coefficient, you calculate the number of atoms by multiplying the coefficient with the subscript. For example, once you put the number 3 in front of CO2, you now have 3*2=6 Os from the CO2. .

IMPORTANT: when balancing equations, only change the coefficients in front of the molecules, never the little subscripts inside the molecules. The coefficients merely change the amount (number) of molecules, but changing the subscript changes the identity of the molecule. You can change the quantity, but not the identity.

After balancing the equation, you get:

C3H8    +  5O2-------> 3CO2   +   4H2O

Let’s say the question next asks you to calculate the number of moles of CO2 when 15 moles of O2 are used. These kinds of questions can confuse students, but the trick I use in solving these problems is to pretend I am following a cooking recipe. The coefficients are units and the molecules are cooking ingredients.

Here is how it goes:

One ingredient A (C3H8) and five ingredients B (O2), give me three C products (CO2) and four D products (O2), and I can only work in these ratios when using this recipe.  What if I have 15 ingredients B (O2)? How many ingredients A would I need and how many C and D products would I make? 

Well, if I were cooking, I would multiply everything with 3 in my recipe, so I would need 3 ingredients A (C3H8), and I would make nine C products (CO2) and twelve D products (O2). So the new equation would look like:

3C3H8    +  15O2-------> 9CO2   +   12H2O

With the same cooking analogy, changing the coefficients changes the quantity of food you are cooking with, but changing the subscripts changes the kind of food (which you should not do when following a recipe). 

Remember, what I suggested here is just one way of approaching these problems. A different way might work better for you. For additional homework help with chemistry whether for high school or college academics, talk to a chemistry tutor.

Entela earned her PhD in Molecular and Cellular Biology (Harvard) and her BS in Biochemistry and Molecular Biology (Clark, summa cum laude, Phi Beta Kappa). Currently, she is the instructor for the Introductory Biology Class at MIT.

Comments

topicTopics
academics study skills MCAT medical school admissions SAT expository writing college admissions English MD/PhD admissions strategy writing LSAT GMAT GRE physics chemistry math biology graduate admissions academic advice ACT interview prep law school admissions test anxiety language learning premed MBA admissions career advice personal statements homework help AP exams creative writing MD study schedules test prep computer science Common Application summer activities history mathematics philosophy organic chemistry secondary applications economics supplements research 1L PSAT admissions coaching grammar law psychology statistics & probability legal studies ESL CARS SSAT covid-19 dental admissions logic games reading comprehension engineering USMLE calculus PhD admissions Spanish mentorship parents Latin biochemistry case coaching verbal reasoning DAT English literature STEM excel medical school political science skills AMCAS French Linguistics MBA coursework Tutoring Approaches academic integrity chinese letters of recommendation Anki DO Social Advocacy admissions advice algebra art history artificial intelligence astrophysics business cell biology classics diversity statement gap year genetics geometry kinematics linear algebra mechanical engineering mental health presentations quantitative reasoning study abroad technical interviews time management work and activities 2L DMD IB exams ISEE MD/PhD programs Sentence Correction adjusting to college algorithms amino acids analysis essay athletics business skills careers cold emails data science dental school finance first generation student functions graphing information sessions international students internships logic networking poetry resume revising science social sciences software engineering tech industry trigonometry writer's block 3L AAMC Academic Interest EMT FlexMed Fourier Series Greek Health Professional Shortage Area Italian Lagrange multipliers London MD vs PhD MMI Montessori National Health Service Corps Pythagorean Theorem Python Shakespeare Step 2 TMDSAS Taylor Series Truss Analysis Zoom acids and bases active learning architecture argumentative writing art art and design schools art portfolios bacteriology bibliographies biomedicine brain teaser campus visits cantonese capacitors capital markets central limit theorem centrifugal force chemical engineering chess chromatography class participation climate change clinical experience community service constitutional law consulting cover letters curriculum dementia demonstrated interest dimensional analysis distance learning econometrics electric engineering electricity and magnetism escape velocity evolution executive function freewriting genomics harmonics health policy history of medicine history of science hybrid vehicles hydrophobic effect ideal gas law immunology induction infinite institutional actions integrated reasoning intermolecular forces intern investing investment banking lab reports linear maps mandarin chinese matrices mba medical physics meiosis microeconomics mitosis mnemonics music music theory nervous system neurology neuroscience object-oriented programming office hours operating systems