10.13 确定分子公式

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  确定分子公式

  

  

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  How can you determine the differences between these two molecules?
  ::您如何确定这两个分子之间的差别 ?

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  Above we see two : glucose and sucrose.  Sucrose is almost exactly twice the size of glucose, although their empirical formulas are very similar.  Some people could distinguish them on the basis of taste, but it’s not a good idea to go around tasting chemicals. The best way is to determine the molecular weights – this approach allows you to easily tell which is which.
  ::以上我们可以看到两点:葡萄糖和sucrose。 Sucrosse几乎完全是葡萄糖的两倍,尽管它们的经验公式非常相似。 有些人可以根据品味来区分它们,但绕着化学物质去尝试并不是个好主意。 最好的方法是确定分子重量 — — 这种方法可以很容易地让您知道哪个是哪个。

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  Molecular Formulas
  ::分子公式

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  give the kind and number of atoms of each present in a molecular compound.  In many cases, the molecular formula is the same as the .  The molecular formula of methane is CH ₄ and because it contains only one carbon , that is also its empirical formula.  Sometimes, however, the molecular formula is a simple whole-number multiple of the empirical formula.  Acetic acid  is an organic that is the main component of vinegar.  Its molecular formula is C ₂ H ₄ O ₂ .  Glucose is  a simple sugar that cells use as a primary source of energy .  Its molecular formula is C ₆ H ₁₂ O ₆ .  The structures of both molecules are shown in the figure below.  They are very different compounds, yet both have the same empirical formula of CH ₂ O.
  ::分子化合物中每个原子的种类和数量给定。 在许多情况下, 分子式与 分子式相同。 甲烷的分子式是CH4, 甲烷的分子式是CH4, 因为它只包含一个碳, 这也是它的经验公式。 然而, 有时, 分子式是经验公式的简单整数倍。 乙酸是醋酸的主要成分之一的有机体。 其分子式是C2H4O2。 葡萄糖是一种简单的糖, 细胞用来作为主要的能源来源。 其分子式是 C6H12O6。 两种分子的结构在下图中显示。 它们都是非常不同的化合物, 但两者都有相同的CH2O的经验公式。

  

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  Acetic acid (left) has a molecular formula of C ₂ H ₄ O ₂ , while glucose (right) has a molecular formula of C ₆ H ₁₂ O ₆ . Both have the empirical formula CH ₂ O.
  ::乙酸(左)的分子式为C2H4O2,而葡萄糖(右)的分子式为C6H12O6。 这两种物质都有实验式CH2O。

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  Empirical formulas can be determined from the of a compound.  In order to determine its molecular formula, it is necessary to know the of the compound.  Chemists use an instrument called a mass spectrometer to determine the molar mass of compounds.  In order to go from the empirical formula to the molecular formula, follow these steps:
  ::为了确定化合物的分子式,必须了解化合物的分子式。化学家使用一种称为质量光谱仪的仪器来确定化合物的摩尔质量。为了从实验式到分子式,请遵循这些步骤:

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  1. Calculate the empirical formula mass (EFM) , which is simply the molar mass represented by the empirical formula.
     ::计算实证公式质量(EFM),即实证公式所代表的摩尔质量。
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  2. Divide the molar mass of the compound by the empirical formula mass. The result should be a whole number or very close to a whole number.
     ::将化合物的摩尔质量除以实证公式质量。 结果应该是整数或非常接近整数。
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  3. Multiply all the subscripts in the empirical formula by the whole number found in step 2. The result is the molecular formula.
     ::将实验公式中的所有下标乘以第2步中发现的全部数字,结果就是分子公式。

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  Sample Problem One: Determining the Molecular Formula of a Compound
  ::样本问题一:确定化合物的分子公式

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  The empirical formula of a compound of boron and hydrogen is BH ₃ .  Its molar mass is 27.7 g/mol.  Determine the molecular formula of the compound.
  ::和氢化合物的经验公式是BH3,其摩尔质量为27.7克/摩尔,确定化合物的分子公式。

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  Step 1: List the known quantities and plan the problem.
  ::第1步:列出已知数量并规划问题。

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  Known
  ::已知已知

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  • empirical formula = BH ₃
    ::经验公式 = BH3
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  • molar mass = 27.7 g/mol
    ::摩尔质量=27.7克/摩尔

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  Unknown
  ::未知

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  • molecular formula = ?
    ::分子公式=?

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  Steps to follow are outlined in the text.
  ::案文概述了应采取的步骤。

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  Step 2: Calculate.
  ::第2步:计算。

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  1. The empirical formula mass (EFM) = 13.84 g/mol
  ::1. 实证公式质量(EFM)=13.84克/摩尔

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  2. $\begin{array}{r}\frac{\text{molar mass}}{\text{EFM}}=\frac{27.7}{13.84}=2\end{array}$
  ::2. 摩质EFM=27.713.84=2

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  3. $\begin{array}{r}{\text{BH}}_3\times 2={\text{B}}_2{\text{H}}_6\end{array}$
  ::3. H3x2=B2H6

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  The molecular formula of the compound is B ₂ H ₆ .
  ::化合物的分子式为B2H6。

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  Step 3: Think about your result.
  ::步骤3:想想你的结果。

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  The molar mass of the molecular formula matches the molar mass of the compound.
  ::分子公式的摩尔质量与化合物的摩尔质量相符。

   

   

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  Summary
  ::摘要

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  • A procedure is described that allows the calculation of the exact molecular formula for a compound.
    ::描述一种程序,可以计算化合物的确切分子公式。

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  Review
  ::回顾

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  1. What is the difference between an empirical formula and a molecular formula?
     ::经验公式和分子公式之间有什么区别?
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  2. In addition to the elemental analysis, what do you need to know to calculate the molecular formula?
     ::除了元素分析之外,你还需要知道什么来计算分子公式?
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  3. What does the empirical formula mass tell you?
     ::实验公式质量告诉你什么?