IME/ITA ⇒ (ITA-1961) Uma mistura de um dos pentanos C5H12

[EngHaw]

Parte III – Questões do Tipo Múltipla Escolha

39 – Uma mistura de um dos pentanos C5H12 com um dos pentenos C5H10 reage completamente com 40,0 mL de uma solução 0,50 formal de bromo.
A relação de moles de penteno/moles pentano na mistura original e:
A – 1 : 2;
B – 2 : 1;
C – 1 : 3;
D – 4 : 1;
E – Faltam dados para responder à pergunta.
F - Nenhuma das respostas acima.

Resposta

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Gabarito Oficial INDISPONÍVEL

Concentração Formal: Uma concentração formal ocorre quando a concentração de uma substância é medida como o número de moles da substância encontrada em um litro de solução. A unidade para concentrações formais é formal e o símbolo para concentrações formais é F e representa o número de moles por litro.
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Molarity and Formality
Both molarity and formality express concentration as moles of solute per liter of solution; however, there is a subtle difference between them. Molarity is the concentration of a particular chemical species. Formality, on the other hand, is a substance’s total concentration without regard to its specific chemical form. There is no difference between a compound’s molarity and formality if it dissolves without dissociating into ions. The formal concentration of a solution of glucose, for example, is the same as its molarity.
For a compound that ionizes in solution, such as CaCl2, molarity and formality are different. When we dissolve 0.1 moles of CaCl2 in 1 L of water, the solution contains 0.1 moles of Ca2+ and 0.2 moles of Cl–. The molarity of CaCl2, therefore, is zero since there is no undissociated CaCl2 in solution; instead, the solution is 0.1 M in Ca2+ and 0.2 M in Cl–. The formality of CaCl2, however, is 0.1 F since it represents the total amount of CaCl2 in solution. This more rigorous definition of molarity, for better or worse, largely is ignored in the current literature, as it is in this textbook. When we state that a solution is 0.1 M CaCl2 we understand it to consist of Ca2+ and Cl– ions. We will reserve the unit of formality to situations where it provides a clearer description of solution chemistry.
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