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Exercício de formula quimica
Exercício:
A fórmula molecular do ácido acético, principal componente do vinagre, é C2H4O2. Assinale a alternativa que apresenta a fórmula empírica e porcentual em massa dessa molécula.
Dados: C = 12 g/mol; H = 1 g/mol; O = 16 g/mol.
Para achar a fórmula empírica você precisa dividir toda a molécula pelo menor número de mols em que todos os valores sejam os menores inteiros possíveis.
Neste caso, se você dividir toda a molécula por 2, ela ainda terá valores inteiros e que serão os menores possíveis:
C2H4O2/2 = CH2O
Para achar a fórmula percentual você precisa fazer uma analogia entre um número e uma porcentagem. Em 99,9% dos casos, a analogia que você vai fazer é entre a massa molar e a porcentagem.
A questão te informou as massas molares de cada átomo da molécula, então você pode calcular a massa molar total da molécula, basta multiplicar as massas molares de cada átomo pela sua “quantidade” na molécula, logo:
C2 = 12 x 2 = 24g/mol
H4 = 1 x 4 = 4g/mol
O2 = 16 x 2 = 32g/mol
MMtotal = 24 + 4 + 32 = 60g/mol
Com essas informações, vamos fazer a nossa razão e proporção (regra de 3) para CADA ÁTOMO
Para o Carbono:
60g/mol —- 100%
24g/mol —- x%
x = 40%
Para o Hidrogênio:
60g/mol —- 100%
4g/mol —– y%
y = 6,7% (aproximadamente)
Para o Oxigênio:
Como falta apenas um resultado, você tem duas opções:
1) Faz a regra de 3 como nos casos anteriores
2) Só diminuir 100 pela soma dos dois resultados anteriores
Então,
Método 1)
60g/mol —- 100%
32g/mol —- z%
z = 53,3% (aproximadamente)
Método 2)
100% – (40+6,7)% => 100% – 46,7% = 53,3%
Então, aplicando todos os valores, a sua fórmula percentual fica assim:
C40%H6,7%O53,3%
R:
Fórmula empírica: CH2O
Fórmula Centesimal/Percentual: C40%H6,7%O53,3%
Espero que eu tenha ajudado e tenha sido claro na explicação. =)
Abraços.
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