Sciencemadness Discussion Board

how often do I need to do calculate AMUs/Moles manually ?

rodster - 1-8-2024 at 01:00

when converting units like moles and AMUs to and from grams and other units, how often is this done by manual calculation ?

j_sum1 - 1-8-2024 at 03:18

In practice I am converting between moles and grams all the time - nearly every reactant I use. Also products, because I will usually scale my reactions according to the amount of product I am aiming for.

It is like tying your shoelaces. You learn how to do it and then you do it constantly, without thinking, any time you need to.

I often do calculations in my head or on the whiteboard in my lab. But I slfo use a calculator. Not sure what you mean by "manually", but it would be rare to use a computer. There just isn't a need.

Sulaiman - 1-8-2024 at 07:18

Quote: Originally posted by rodster  
...how often is this done by manual calculation ?
very very often :)
Typically I first write down the chemical equation for the planned reaction,
Then, for each molecule in the equation, I look at the relevant wikipedia article,
First you need to check your starting materials:
Solids: anhydrous, or how many waters of hydration are attached to the molecule, sometimes I need to rewrite the equation.
Now you can use molecular weights in your equation, grams per mole.
Liquids: you need to dispense either by weight or volume :
so you need to know the weight% or molarity of the liquid(s).
Now you have an equation with weights and/or volumes required for the reactants and products.
Usually I multiply or divide by a convenient number to work at my chosen scale (eg weight or volume of product required)
While in the Wikipedia articles check toxicity, preparation and reactions sections, useful info such as boiling or melting points worth reading.

At first it IS confusing, but it gets easy very quickly.
Practice makes it easier, but often not perfect .

rodster - 1-8-2024 at 07:56

well to be clear I meant manual as in without calculators.

Also to the person saying they use wikipedia, that's literally a source every teacher I've known advised against using.

EF2000 - 1-8-2024 at 09:15

Quote: Originally posted by rodster  
well to be clear I meant manual as in without calculators.

Also to the person saying they use wikipedia, that's literally a source every teacher I've known advised against using.

What is the reason to calculate without a calculator? (Unless on the final math exam, horrorful experience)

I too can say a lot of bad things about Wikipedia (they banned me for political reasons), but the data in infoboxes in chemical articles is generally very reliable. It comes from databases like PubChem. If Wikipedia says that, for example, nickel tetracarbonyl has molar mass of 170.73 g/mol and shows a lot of scary pictograms, it's highly likely to be true. It's not like modern history or politics (where WP really is often not reliable).

You can get yourself a handbook, like CRC Handbook, but you will soon find that handbooks tend to be hard to navigate. The needed property is always in the another table some 300 pages later. While the data in WP article is few clicks away and absolutely the same (since they took it from handbook).

rodster - 1-8-2024 at 14:17

I am sure some are good overall but I just find the constant edits interesting, if one checks the edit history of any article. especially when it's not a topic that has like a ton of new info coming out regularly. It's just a sign the wiki editors can never agree on certain things, even on the "good" articles.

Anyway manual calculation was just part of some learning material I was going over so was wondering how relevant it was.

Plus as you indicated it may be expected by academic institutions on tests or exams, though don't know from personal experience.


bnull - 1-8-2024 at 16:20

Quote: Originally posted by rodster  
when converting units like moles and AMUs to and from grams and other units, how often is this done by manual calculation ?

When I'm lazy or in a hurry, my fx-82MS (good old fella has been running on the same battery since 2016 :cool: ) handles the numbers. Otherwise, it's pen and paper. During college, I did trigonometric functions (circular, hyperbolic, inverse) and roots on the calculator and the rest by hand.

Manual calculation is a good exercise, helps to keep your brain healthy. After months and months of calculus at college, you begin to forget how much is 2 plus 2, how to sum fractions, how to calculate squares or use the powers of ten. It's like practicing scales on the guitar.

And you learn better by writing on a physical medium, not to mention that it also helps to keep your writing legible.
Quote: Originally posted by rodster  
Anyway manual calculation was just part of some learning material I was going over so was wondering how relevant it was.

It is really relevant. You don't need electricity, batteries, a screen, sometimes not even paper. Don't skip it. Keep the calculators away the most you can.

j_sum1 - 1-8-2024 at 18:59

Mental arithmetic is a good skill for a very large number of reasons.
Principally because it keeps your head closer to the problem and ensures you get meaningful answers.
  • You usually get an early estimate (which is useful and helps error detection)
  • You recognise patterns more readily
  • It lowers the burden of interpreting the calculation result -- because your brain is already in that space

    Plenty of other reasons as well: alertness, mental dexterity, aids memory, promotes learning of new concepts, etc.
    I think it is good practice. But I do dive for the calculator for speed, accuracy or for analytical work when it is needed. No one needs to divide by a five significant decimal number by hand. We have centuries of mathematicians developing tables and tools to avoid this. Even so, it is my practice to perform a mental estimate to accompany the calculation.

    Sulaiman - 2-8-2024 at 00:21

    Quote: Originally posted by rodster  
    well to be clear I meant manual as in without calculators.

    Also to the person saying they use wikipedia, that's literally a source every teacher I've known advised against using.

    A few practical considerations:
    A) who does not have an electronic calculator? (physical or app)
    B) how accurate and precise is your system of measurements?
    There is little to be gained by over-precise calculations
    C) commonly one of the reactants is expensive and/or hard to obtain,
    so we commonly add excess of the cheaper reactant(s) to get the best yield.
    Often a reaction will proceed differently if X is added to Y instead of Y added to X, occasionally the end results can differ.

    Use whatever data sources you prefer. Opinions may vary.
    I have learned a lot of chemistry via Wikipedia,
    the references sections are a good start to any chemical journey. I'm sure there are better,but I like Wikipedia.
    The main thing is
    Do some basic research before performing an unfamiliar reaction
    or asking questions.

    rodster - 2-8-2024 at 01:03

    Let's just be clear for a second, I was responding to someone's question who asked me why a calculator wouldn't be used (EF2000)

    So some of the commentators here may want to direct their responses to them.

    [Edited on 2-8-2024 by rodster]