POJ 2006：Litmus Test 化学公式

Litmus Test

Time Limit: 1000MS		Memory Limit: 30000K
Total Submissions: 1709		Accepted: 897

Description

The pH scale measures the concentration of protons (H⁺) in a solution and, therefore, its acidity or alkalinity. The pH value of a solution is a number between 0 and 14; it is less than 7 if the solution is acidic, greater than 7 if the solution
is basic, and 7 if it is neutral. 



The formula for calculating pH is 

pH = -log₁₀ [H⁺]


where [H⁺] is the concentration of protons measured in moles per litre. 



To calculate the pH value of an acid, one has to determine the concentration of protons in the solution. When an acid is dissolved in water, an equilibrium is reached and is governed by the equation 

K_a = [H⁺] [acid ions] / [acid]


where K_a is the acidity constant (known for each acid), [acid ions] is the concentration of the acid ions that have dissolved, and [acid] is the concentration of the undissolved acid. Before the acid is added, both [H⁺] and [acid ions]
are assumed to be 0. 

For example, the acidity constant of methanoic acid is 1.6 * 10^-4. Dissolving one mole of acid molecules results in one mole of H⁺ and one mole of acid ions. If the initial concentration of the methanoic acid is 0.1 moles/L and x moles
of acid are dissolved (per liter), then the final concentration at equilibrium would be 0.1 - x moles/L for the acid and x moles/L for H⁺ and the acid ions.

Input

The input consists of a number of test cases. Each test case contains 4 numbers on a line: two positive floating-point numbers specifying the acidity constant K_a and the original concentration of the acid (in moles/liter) added to the water, as well
as two positive integers m and n indicating that each mole of acid molecules is dissolved into m moles of H⁺ ions and n moles of acid ions. The floating-point numbers are specified in scientific notation as shown below. The input is terminated with
a line containing four zeros. 

Output

For each test case, print on a line the pH value of the solution, rounded to 3 decimal places.

Sample Input

1.6e-04 1.0e-01 1 1
1.6e-04 1.0e-01 4 1
1.5e-05 5.0e-02 1 2
0 0 0 0

Sample Output

2.407
2.101
3.216

这个题目的一个收获是我发现指数形式也可以直接使用cin输入，还有log10这个函数。

根据题目给出的第二个公式得到 Ka = mx*nx / ori_con-x，解关于x的方程。最终要转换成-log₁₀ x的形式。

代码：

#include <iostream>
#include <algorithm>
#include <cmath>
#include <vector>
#include <string>
#include <cstring>
#pragma warning(disable:4996)
using namespace std;

int main()
{
	double ka;
	double ori_con;
	int m;
	int n;

	while(1)
	{
		cin>>ka;
		cin>>ori_con;
		cin>>m;
		cin>>n;

		if(ka+ori_con+m+n==0)
			break;
		double x=(sqrt(ka*ka+4*ka*ori_con*m*n)-ka)/(2*n);

		printf("%.3f\n",-log10(x));
	}
	return 0;
}

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