10.8 气体密度

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  气体密度

  

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  Why does carbon dioxide sink in air?
  ::为什么二氧化碳会在空气中沉没?

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  When we run a reaction to produce a , we expect it to rise into the air.  Many students have done experiments where gases such as hydrogen are formed.  The gas can be trapped in a test tube held upside-down over the reaction.  Carbon dioxide, on the other hand, sinks when it is released.  Carbon dioxide has a density greater that air, so it will not rise like these other gases would.
  ::当我们产生一种反应时,我们期望它会升到空气中。许多学生在形成氢等气体的地方进行了实验。气体可以被困在试验管里,反倒压住反应。另一方面,二氧化碳在释放时会沉入水中。二氧化碳的密度比空气要高,因此不会像其他气体那样上升。

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  Gas Density
  ::气体密度

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  As you know, density is defined as the mass per unit volume of a substance .  Since gases all occupy the same volume on a per mole basis, the density of a particular gas is dependent on its .  A gas with a small molar mass will have a lower density than a gas with a large molar mass.  Gas densities are typically reported in g/L.  Gas density can be calculated from molar mass and molar volume .
  ::如你所知,密度被定义为物质的单位体积质量。由于气体每摩尔的体积相同,特定气体的密度取决于其密度。小摩尔质量气体的密度将低于大摩尔质量气体的密度。气体密度通常以克/升报告。气体密度可以从摩尔质量和摩尔体积计算。

  

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  Sample Problem One: Gas Density
  ::抽样问题一:气体密度

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  What is the density of nitrogen gas at STP?
  ::保护受威胁人民协会的氮气密度是多少?

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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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  • N ₂ = 28.02 g/mol
    ::N2=28.02克/摩尔
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  • 1 mol = 22.4 L
    ::1毫毫升=22.4升

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  Unknown
  ::未知

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  • density = ? g/L
    ::密度=? g/L

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  Molar mass divided by molar volume yields the gas density at STP.
  ::摩尔质量除以摩尔体积,产生STTP的气体密度。

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  Step 2: Calculate.
  ::第2步:计算。

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  $$\begin{array}{r}\frac{28.02\text{g}}{1\text{mol}}\times \frac{1\text{mol}}{22.4\text{L}}=1.25\text{g}/\text{L}\end{array}$$

  
  ::28.02 g1 molx1 mol22.4 L=1.25 g/L

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  When set up with a conversion factor , the mol unit cancels, leaving g/L as the unit in the result.
  ::当设置一个换算系数时, mol 单位会取消,结果将g/L作为单位。

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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 nitrogen is slightly larger than molar volume, so the density is slightly greater than 1 g/L.
  ::氮的摩尔质量略大于摩尔体积,因此密度略高于1克/升。

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  Alternatively, the molar mass of a gas can be determined if the density of the gas at STP is known.
  ::或者,如果知道受威胁人民协会的气体密度,就可以确定气体的摩尔质量。

   

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  Sample Problem Two: Molar Mass from Gas Density
  ::抽样问题二:气体密度的摩尔质量

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  What is the molar mass of a gas whose density is 0.761 g/L at STP?
  ::高密度为0.761克/升的气体的摩尔质量是多少?

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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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  • Density of gas = 0.761 g/L
    ::气体密度=0.761克/升
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  • 1 mol = 22.4 L
    ::1毫毫升=22.4升

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  Unknown
  
  ::未知

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  • molar mass = ? g/mol
    ::摩尔质量=? g/摩尔

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  Molar mass is equal to density multiplied by molar volume.
  ::摩尔质量等于密度乘以摩尔体积。

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  Step 2: Calculate.
  ::第2步:计算。

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  $$\begin{array}{r}\frac{0.761\text{g}}{1\text{L}}\times \frac{22.4\text{L}}{1\text{mol}}=17.0\text{g}/\text{mol}\end{array}$$

  
  ::0.761 g1 Lx22.4 L1 mol=17.7 g/mol

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

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  Because the density of the gas is less than 1 g/L, the molar mass is less than 22.4.
  ::由于气体密度小于1克/升,摩尔质量小于22.4。

   

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  Summary
  ::摘要

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  • Calculations are described showing conversions between molar mass and density for gases.
    ::所描述的计算数字显示了摩尔质量和气体密度之间的转换。

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  Review
  ::回顾

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  1. How is density calculated?
     ::密度是如何计算的?
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  2. How is molar mass calculated?
     ::摩尔质量是如何计算的?
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  3. What would be the volume of 3.5 moles of a gas?
     ::气体的3.5摩尔的体积是多少?