Anonymous
Anonymous asked in Science & MathematicsChemistry · 4 weeks ago

# Probably a stupid question but why isn’t atomic weight measured by pounds but instead mass?

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• 2 weeks ago

You could convert grams to pounds.  It just takes a few seconds with Google.

The molar mass of sodium is 23.0 g/mol, or 0.0507 lb/mol.

Scientists use the metric system. Pounds are not metric. Pounds are a measure of force, not mass. If you wanted to use the English system, then the slug is the measure of mass. 23.0 g/mol = 0.001576 slugs per mole.

"A slug is defined as the mass that is accelerated by 1 ft/s2 when a force of one pound (lbf) is exerted on it. One slug is a mass equal to 32.1740 lb (14.59390 kg) based on standard gravity, the international foot, and the avoirdupois pound."

https://en.wikipedia.org/wiki/Slug_(unit)

• 3 weeks ago

I think you're asking why atomic weights can't be expressed in pounds as well as grams. In fact, if you get a chem problem in pounds, just total up the atomic weights in a compound and call it a pound-mole. Then you solve the problem without interconverting grams and pounds

• Jim
Lv 7
3 weeks ago

Mass is used by scientists. Even the US is standardized on the SI metric system.

Pounds would be scoffed at by scientists since they would be so difficult to work with. And they would need to be converted to 'slugs' as the unit of mass.

• Dr W
Lv 7
4 weeks ago

.. "why isn't atomic mass measured by pounds but instead mass?

that doesn't make a lot of sense there Mr/Ms anonymous.

.. (1) atomic mass is the mass of 1 atom

.. (2) "pounds" is ambiguous.  it could mean lbf.. pounds force

.. .....or it could mean lbm... pounds mass

.. (3) there are 454g in 1 lbm and there are 1.66x10^24 amu in 1g.

.. .. . meaning for an average atom of say carbon, you would need

.. .. . to have about 1x10^27 atoms to equal 1 "pound"

.. . . .meaning.. to "measure" atomic mass in "lbm" isn't convenient.

.. (4) atomic mass isn't so much "measured" but rather "calculated"

.. .. . .from other measurements.

************

roger the mole... you wrote

.. "Pounds are not used for anything scientific because no one outside the United States uses pounds, and science is very much an international effort."

I beg to differ.  Those of us in the world of chemical engineering use lbs, lbmoles, tons, tonmole, kg, kgmole etc.. ALL THE TIME ALL OVER THE PLANET.  And I would challenge anyone foolish enough to say "chemists are scientific whilst chemical engineers are not".

*******

ryan..

.. the relative atomic mass of 1 H atom = 1.67x10^-24g = 3.69x10^-27lbm

How many miracles do I need to perform for sainthood?  3?

.. The relative molecular mass of 1 H2 molecule = 2.016amu = 7.38x10^-27lbm

.. The relative atomic mass of 1 O atom = 5.86x10^-26lbm

*******

David.. and.. .Dixon

.. technically "lbs" can refer to lbm (pound mass) or lbf (pound force)

.. (ignoring the British currency)...

the conversion between the two is called gc.. g sub c

since F = ma

.. 1 lbf = 32.174 lbm ft / s^2

.. gc = 32.174 lbm ft / (lbf s^2)

and

.. technically.. when you use 453.54g = 1lb.. that lb is lbm not lbf.. i.e

.. technically... pounds is a unit of mass when you convert from grams

https://en.wikipedia.org/wiki/Pound_(mass)

https://en.wikipedia.org/wiki/Pound_(force)

https://en.wikipedia.org/wiki/Gc_(engineering)

• Amy
Lv 7
4 weeks ago

Every carbon-12 isotope in the universe has the same mass, so it makes sense to describe carbon as having an atomic mass.

Identical atoms can have different weight depending on the gravitational field they happen to be in at any moment. Even across Earth's surface, gravity varies by about 1%.

• Dixon
Lv 7
4 weeks ago

Pounds are a measure of weight (force due to gravity) but weight is irrelevant at the scale of individual atoms, whereas mass is intrinsic to the atom itself.

• ?
Lv 7
4 weeks ago

Technically 'pounds' are a unit of force -- not mass.

There was a habit several decades ago to refer to the "atomic weight" the now the term "atomic MASS" is used to emphasize that science uses mass, not force for these measurements,

Remember --  MASS does not change with position .. 1 gram is 1 gram on earth or on the moon.  But FORCE created by gravity is what weight is.  10 pounds on earth is only about 6 pounds on the moon.

. . . There is a change in the weight of the same object at sea level on the equator compared to its weight on the top of a mountain or even at a different latitude .. near the North pole and weight increases about 1% if I remember correctly.

In chem. we want these measurement to be the same everywhere.

• 4 weeks ago

Pounds are not used by any chemists or physicists because no one outside the United States uses pounds, and those sciences are very much international efforts.

To put it another way, the unit "pound" is not used for same reason that "uckir" is not used.

[Ryan:  Weights of nucleons can certainly be expressed in pounds, but there is no reason to do it -- except to prove it can be done.

The mass of a neutron is 1.67493 × 10^−27 kg, so one neutron weighs

(1.67493 × 10^−27 kg) x (2.20462 lbs/kg) = 3.69258 × 10^-27 lbs ]

• 4 weeks ago

It would be impossible to express the weight of protons and neutrons in an atom in pounds.