CÁLCULOS ENEM 40
Autor: PROF. ROSSETTI | Publicado: 2017-08-03T08:55:00-03:00


(ENEM
2015/2) A cafeína é um alcaloide, identificado como&nbsp;

1,3,7 – trimetilxantina (massa
molar igual a 194 g/mol), cuja estrutura química contém uma unidade de purina,
conforme representado. Esse alcaloide é encontrado em grande quantidade nas
sementes de café e nas folhas de chá-verde. Uma xícara de café contém, em média,
80 mg de cafeína.



Considerando
que a xícara descrita contém um volume de 200 mL de café, a concentração, em
mol/L, de cafeína nessa xícara é mais próxima de



<!--[if !supportLists]-->&nbsp; &nbsp; &nbsp; &nbsp; &nbsp;a)&nbsp;&nbsp; <!--[endif]-->0,0004&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; b) 0,002&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; c)&nbsp;
0,4&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; d) 2&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; e) 4





RESOLUÇÃO POR ANÁLISE DIMENSIONAL



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       m:val="p"/> <span
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    lang=EN-US style='font-size:20.0pt;font-family:"Cambria Math","serif";
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       m:val="roman"/>1 mol<span
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     style='mso-spacerun:yes'>  194 g<span
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RESPOSTA:
letra B