Name: chemical formula mass and percent composition
Uploaded: 2020-09-03T13:38:26.000Z
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Formula Mass and Mole Concepts of Compounds

Najmniejszą reprezentatywną jednostką związku jest cząsteczka (lub jednostka wzoru), przedstawiona wzorem chemicznym.

Masy atomowe wszystkich atomów we wzorze chemicznym, jeśli są dodane, dają masę cząsteczkową (lub masę wzoru) związku, mierzoną w jednostce masy atomowej.

Na przykład, aby znaleźć masę cząsteczkową sacharozy lub cukru stołowego, masę atomową każdego pierwiastka mnoży się przez liczbę atomów odpowiedniego pierwiastka. Suma całkowita daje masę cząsteczkową sacharozy.   

Masa wszystkich cząsteczek (lub jednostek wzoru) obecnych w jednym molu związku chemicznego nazywana jest jego masą molową, mierzoną w g/mol.

Masa molowa związku jest liczbowo równa jego masie cząsteczkowej (lub masie wzoru). Na przykład sacharoza o masie cząsteczkowej 342,3 amu będzie miała masę molową 342,3 g/mol.

Znajomość rzeczywistej liczby cząsteczek lub cząstek obecnych w substancji chemicznej ma fundamentalne znaczenie dla zrozumienia jej składu, interakcji i kombinacji. 

Jednak w danej próbce związku liczenie rzeczywistej liczby cząsteczek zwykłymi metodami jest niepraktyczne. Wykorzystując zależność między masą molową, molem i liczbą Avogadro, można obliczyć całkowitą ilość obecnych cząsteczek.

Zgodnie z tą koncepcją masa molowa związku jest równa jednemu molowi związku, stanowiącemu 6,022 × 1023 cząstki. Dlatego 342,3 grama sacharozy zawiera 6,022 × 1023 cząsteczki sacharozy.

Jeśli jednak próbka sacharozy waży na przykład 150 gramów, liczbę cząsteczek można określić za pomocą obliczeń przy użyciu metody mnożenia krzyżowego. Wynik pokazuje, że 150 gramów sacharozy stanowi 2,63 × 1023 cząsteczki sacharozy.   

Zawartość procentową każdego pierwiastka w związku można wyrazić za pomocą wartości procentowej masy. Aby obliczyć, masa pierwiastka w jednym molu związku jest dzielona przez całkowitą masę jednego mola związku i przeliczana na procent.

W praktyce, aby obliczyć procent masowy wodoru w sacharozy, we wzorze uwzględniono całkowitą masę atomów wodoru i całkowitą masę cząsteczek sacharozy. Obliczona wartość pokazuje, że sacharoza zawiera 6,4 procenta masowego wodoru.

pojęcia dotyczące masy i mola związków

<h4>Wzór na masę związków kowalencyjnych</h4>

Wzory chemiczne przedstawiają pierwiastkowy skład substancji. W przypadku związków kowalencyjnych, wzór przedstawia liczbę i rodzaje atomów tworzących pojedynczą cząsteczkę substancji; dlatego masa wzoru może być poprawnie określana jako masa cząsteczkowa. Wzór cząsteczkowy związku kowalencyjnego chloroform (CHCl3) wskazuje, że pojedyncza cząsteczka zawiera jeden atom węgla, jeden atom wodoru i trzy atomy chloru. Średnia masa cząsteczkowa cząsteczki chloroformu jest równa sumie średnich mas atomowych tych atomów:

<img alt="Eq1" src="https://www.jove.com/files/core/CDNSource/lessons/1017/1017_Equations-01.png" style="height: 40px;">

<h4> Wzór na masę związków jonowych</h4>

Związki jonowe składają się z oderwanych kationów i anionów, których proporcje pozwalają uzyskać elektrycznie obojętną materię. Wzór na masę związku jonowego oblicza się w taki sam sposób, jak wzór na masę dla związków kowalencyjnych: poprzez zsumowanie średnich mas atomowych wszystkich atomów we wzorze związku. Wzór związku jonowego nie przedstawia jednak składu odrębnej cząsteczki, dlatego nie można go poprawnie nazwać „masą cząsteczkową”.

Na przykład zwykła sól kuchenna lub chlorek sodu (NaCl) to związek jonowy składający się z kationów sodu (Na+) i anionów chlorkowych (Cl-) połączonych w stosunku 1:1. Wzór na masę tego związku oblicza się poprzez dodanie średnich mas atomowych jego pierwiastków składowych:

<img alt="Eq2" src="https://www.jove.com/files/core/CDNSource/lessons/1017/1017_Equations-02.png" style="height: 40px;">

Przy obliczaniu wzoru na masę związku jonowego stosowano średnie masy obojętnych atomów sodu i chloru, a nie masy kationów sodu i anionów chlorkowych. Chociaż kation sodu ma nieco mniejszą masę niż atom sodu (ponieważ brakuje mu elektronu), różnica ta zostanie zrównoważona przez to, że anion chlorkowy jest nieco bardziej masywny niż atom chloru (ze względu na dodatkowy elektron). Co więcej, masa elektronu jest zaniedbywalnie mała w porównaniu z masą typowego atomu.

<h4>Skład w procentach masowych</h4>

Skład pierwiastkowy związku określa jego tożsamość chemiczną, a wzory chemiczne są najbardziej zwięzłym sposobem przedstawienia tego składu pierwiastkowego. Procent masowy każdego pierwiastka w związku nazywany jest procentem masowym tego konkretnego pierwiastka. Skład procentowy można obliczyć, dzieląc masę każdego pierwiastka przez całkowitą masę związku, a następnie przeliczając go na procent.

Skład procentowy jest przydatny do oceny względnej obfitości danego pierwiastka w różnych związkach o znanych wzorach. O ile znany jest wzór cząsteczkowy lub empiryczny danego związku, skład procentowy można wyprowadzić z mas atomowych lub molowych pierwiastków związku.

Na przykład jedna cząsteczka kwasu azotowego (HNO3) zawiera jeden atom azotu o masie 14,01 u, jeden atom wodoru o masie 1,008 u i trzy atomy tlenu o masie (3 × 16,00 u) = 48,00 u. Wzór na masę kwasu azotowego wynosi zatem (14,01 u + 1,008 u + 48,00 u) = 63,02 u, a jego skład procentowy to:

<img alt="Eq3" src="https://www.jove.com/files/core/CDNSource/lessons/1017/1017_Equations-03.png" style="height: 40px;">
<img alt="Eq4" src="https://www.jove.com/files/core/CDNSource/lessons/1017/1017_Equations-04.png" style="height: 40px;">
<img alt="Eq5" src="https://www.jove.com/files/core/CDNSource/lessons/1017/1017_Equations-05.png" style="height: 40px;">

Pojęcia dotyczące masy i mola związków

Wzór Masa związków kowalencyjnych

Wzory chemiczne reprezentują skład pierwiastkowy substancji. W przypadku związków kowalencyjnych wzór reprezentuje ...

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Kluczowe terminy i pojęcia

chemical formula mass and percent composition

Chemical formula mass and percent composition help describe the makeup of compounds. A chemical formula shows which atoms are present and how many of each atom appear in a substance.

For covalent compounds, the formula shows the atoms in one molecule. That is why formula mass can also be called molecular mass. Chloroform, CHCl3, is one example. Its formula shows one carbon atom, one hydrogen atom, and three chlorine atoms. The molecular mass is the sum of their average atomic masses.

Ionic compounds are different because they contain cations and anions that combine in ratios that make the bulk material electrically neutral. Their formula mass is found the same way, by adding the average atomic masses of the atoms in the formula. Sodium chloride, or table salt, is one example. Its formula, NaCl, shows sodium and chloride in a 1:1 ratio. For ionic compounds, the term molecular mass is not the best choice because the formula does not describe a single molecule.

When calculating formula mass for an ionic compound, neutral atom masses are used instead of ion masses. A sodium cation has slightly less mass than a sodium atom because it has lost an electron. A chloride anion has slightly more mass than a chlorine atom because it has gained an electron. The difference is small, and the mass of an electron is negligibly small compared with the mass of a typical atom.

Percent composition tells how much of a compound’s mass comes from each element. It is found by dividing the mass of each element by the total mass of the compound and converting the result to a percentage. This is useful for comparing the relative amount of an element in compounds with known formulas.

Nitric acid, HNO3, shows how the calculation works. One molecule contains one nitrogen atom with a mass of 14.01 amu, one hydrogen atom with a mass of 1.008 amu, and three oxygen atoms with a total mass of 48.00 amu. Its formula mass is 63.02 amu. From that total, the percent composition of each element can be determined.

Chemical Formula Mass and Percent Composition

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formula mass and mole concepts of compounds

Chemical formula mass and percent composition help describe the makeup of compounds. A chemical formula shows which atoms are present and how many of each atom appear in a substance. For covalent compounds, the formula shows the atoms in one molecule. That is why formula mass can also be called molecular mass. Chloroform, CHCl3, is one example. Its formula shows one carbon atom, one hydrogen atom, and three chlorine atoms. The molecular mass is the sum of their average atomic masses. Ionic...

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Learn how to calculate formula mass for covalent and ionic compounds, then use it to find percent composition with examples like chloroform, salt, and nitric acid.

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