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标题: 请教一道prep数学 [打印本页]

作者: toneere    时间: 2009-11-19 20:39     标题: 请教一道prep数学

The rate of a certain chemical reaction is directly proportional to the square of the concentration of chemical A present and inversely proportional to the concentration of chemical B present. If the concentration of chemical B is increased by 100%, which of the following is closest to the percent change in the concentration of chemical A required to keep the reaction rate unchanged?

 

A 100% decrease

 

B 50% decrease

 

C 40% decrease

 

d 40% increase

 

e 50 % increase

 

correct answer: d

 

我没弄明白题干,请大家指点一下,万分感谢!


作者: taoyeye    时间: 2009-11-20 07:10

 一个化学量同A浓度平方成正比,同B浓度成反比,若B翻倍,要原来的反应速率不变,A浓度当如何?应该变成根号2倍,增加40%左右
作者: antiquarks    时间: 2009-11-24 07:01

化学反映的速率跟A物质的浓度的平方成正比,跟B物质的浓度成反比,现在B的浓度增长了100%,问A的浓度增长多上才能保证反映速率不变。

设原来速率为r,原来B的浓度为b,原来A的浓度为a,所求A的浓度为A,那么列出方程:
r=k(a^2)/(tb)=k(A^2)/(t2b),其中k和t为自设定的比率,常数。

解方程得:A=(根号2)*a,所以选D


作者: toneere    时间: 2009-11-25 06:55

谢谢~~~




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