percent by mass

(noun)

The fraction, by weight, of one element of a compound.

Related Terms

Examples of percent by mass in the following topics:

  • Percent Composition of Compounds

    • The percent composition (by mass) of a compound can be calculated by dividing the mass of each element by the total mass of the compound.
    • Another convenient way to describe atomic composition is to examine the percent composition of a compound by mass.
    • Percent composition is calculated from a molecular formula by dividing the mass of a single element in one mole of a compound by the mass of one mole of the entire compound.
    • Mass percent H in butane: $\frac{10.079\:g\:H}{58.123\:g\:butane} \cdot 100$ = 17.3% H
    • Translate between a molecular formula of a compound and its percent composition by mass

  • Average Atomic Mass

    • The average atomic mass of an element is the sum of the masses of its isotopes, each multiplied by its natural abundance.
    • By adding together the number of protons and neutrons and multiplying by 1 amu, you can calculate the mass of the atom.
    • The average atomic mass of an element is the sum of the masses of its isotopes, each multiplied by its natural abundance (the decimal associated with percent of atoms of that element that are of a given isotope).
    • To calculate the average mass, first convert the percentages into fractions (divide them by 100).
    • To calculate the average atomic mass, multiply the fraction by the mass number for each isotope, then add them together.

  • Post-Cambrian Evolution and Mass Extinctions

    • The post-Cambrian era was characterized by animal evolution and diversity where mass extinctions were followed by adaptive radiations.
    • The end of the Permian period (and the Paleozoic Era) was marked by the largest mass extinction event in Earth's history, a loss of roughly 95 percent of the extant species at that time.
    • Another mass extinction event occurred at the end of the Cretaceous period, bringing the Mesozoic Era to an end.
    • In the following Cenozoic Era, mammals radiated into terrestrial and aquatic niches once occupied by dinosaurs.
    • Differentiate among the causes of mass extinctions and their effects on animal life

  • Mass Spectrometry to Measure Mass

    • Mass spectrometers separate compounds based on a property known as the mass-to-charge ratio: the mass of the atom divided by its charge.
    • A mass analyzer, which sorts the ions by mass by applying electromagnetic fields
    • All other ions will be lost by collision with the rods.
    • Chloride atoms and ions come in two isotopes, with masses of approximately 35 amu (at a natural abundance of about 75 percent) and approximately 37 amu (at a natural abundance of about 25 percent).
    • The ions are separated in an analyze by magnetic fields.

  • Mole Fraction and Mole Percent

    • We can easily convert mole percent back to mole fraction by dividing by 100.
    • First, we convert this volume to a mass by using the density of water (1.00 g/mL), and then we convert this mass to moles of water:
    • The number of moles for each is found by dividing its mass by its respective molecular weight.
    • The mole fraction can also be calculated from a mass percent.
    • What is the mole fraction of cinnamic acid that has a mass percent of 50.00% urea in cinnamic acid?

  • Biodiversity Change through Geological Time

    • Biodiversity has been affected by five mass extinction periods, which greatly influenced speciation and extinction rates.
    • During this period, about 85 percent of marine species (few species lived outside the oceans) became extinct.
    • The main hypothesis for its cause was a period of glaciation followed by warming.
    • Estimates predict that 96 percent of all marine species and 70 percent of all terrestrial species were lost.The causes for this mass extinction are not clear, but the leading suspect is extended and widespread volcanic activity that led to a runaway global-warming event.
    • These researchers hypothesized that this iridium spike was caused by an asteroid impact that resulted in the K–Pg mass extinction.

  • Weight

    • Mathematically, the weight of an object (W) can be found by multiplying its mass (m) by the acceleration due to gravity (g): $W=M\cdot g$.
    • In fact, the greatest percent difference in the value of the acceleration due to gravity on Earth is 0.5%.
    • It is important to note that the apparent weight of an object (i.e., the weight of an object determined by a scale) will vary if forces other than gravity are acting upon the object .
    • For example, if you weigh a given mass underwater you will find a different result than if you weigh that mass in air.
    • A spring scale measures weight by finding the extent to which a spring is compressed.This is proportional to the force that a mass exerts on the scale due to its weight.

  • Structural Determination

    • For example, the most abundant isotope of carbon, C-12, is invisible to NMR, whereas the minor isotope C-13 is NMR active, but only comprises 1.1 percent of a given sample of carbon.
    • By replacing C-12 in a molecule with C-13, NMR analysis of that position is greatly enhanced.
    • If a fragment does not contain the isotope, then its mass will not be changed.
    • The reactant is 'labeled' by replacing specific atoms with their isotope.
    • Mass spectrometry and nuclear magnetic resonance detect the difference in an isotope's mass, while infrared spectroscopy detects the difference in the isotope's vibrational modes.

  • Alpha Decay

    • In alpha decay an atomic nucleus emits an alpha particle and transforms into an atom with smaller mass (by four) and atomic number (by two).
    • As the result of this process, the parent atom transforms ("decays") into a new atom with a mass number smaller by four and an atomic number smaller by two.
    • Alpha particles have a typical kinetic energy of 5 MeV (approximately 0.13 percent of their total energy, i.e., 110 TJ/kg) and a speed of 15,000 km/s.
    • Most of the helium produced on Earth (approximately 99 percent of it) is the result of the alpha decay of underground deposits of minerals containing uranium or thorium.
    • An atomic nucleus emits an alpha particle and thereby transforms ("decays") into an atom with a mass number smaller by four and an atomic number smaller by two.

  • Eratosthenes' Experiment

    • By Earth's gravity, the mass hung straight down; the line of the string established a "vertical" at that geographic location.
    • The mass hung in one well at Syrene showed something special.
    • Thus, supposing 3963 miles is the actual radius, the percent error of Eratosthenes' measurement was:
    • $Percent \; Error = \left(\frac{3979 - 3963}{3963}\right) \times 100\% \cong 0.4\%$
    • Thus his measurement of the Earth circumference (some 2000 years ago) was in error: Less than actual by only one-tenth of a percent.