/* --------------------------------------------------------------------- 
// Figure 9.13:  Eliminating the global variable.
// Figure 9.14:  Functions for gas models after global elimination.
// --------------------------------------------------------------------- 
*/
#include "tools.h"           
                                 
#define R       8314            /* universal gas constant */ 
        
float ideal  ( float v, float temp );  
float vander ( float v, float temp );   

void main( void )                                                                           
{   
    float temp;                 /*  temperature of CO gas */
    float m;                    /*  mass of CO gas, in kilograms  */   
    float vol;                  /*  tank volume, in cubic meters  */   
    float vmol;                 /*  molar specific volume    */        
    float pres;                 /*  pressure (to be calcualted)   */

    banner();                                                       
    printf( "\n Input the temperature of CO gas (Kelvin degrees):  " );
    scanf( "%g",   &temp );                                             
    printf( "\n The mass of the gas (kg) is: " );                    
    scanf( "%g", &m );                                                   
    printf( "\n The tank volume (cubic m) is: " );                  
    scanf( "%g", &vol );  
                                               
    vmol = 28.011 * vol / m;                /* molar volume of CO gas  */
    pres = ideal( vmol, temp );             /* pressure; ideal gas model */     
    printf( "\n The ideal gas at %.3g K  has a pressure of "        
             "%.3g kPa \n", temp, pres );                             
    pres = vander ( vmol, temp );           /* pressure; Van der Waal's model */         
    printf( " Van der Waal's gas at %.3g K has a pressure of "
             "%.3g kPa \n\n", temp, pres ); 
    bye();
}

/* ----------------------------------------------------------------------  
**  Pressure of CO gas in a tank, using the ideal gas equation PV = RT. 
*/  
float 
ideal( float v, float temp )                                                            
{   
    float p;                        /* LOCAL variable DECLARATION */
                        
    p = R * temp / v;               /* pressure in  Pascals  */                            
    return p / 1000.0;              /* pressure in kilo Pascals (kPa) */                 
}

/* ----------------------------------------------------------------------  
**  Pressure of CO gas in a tank, using Van der Waal's equation,            
**  P = RT/(v-b) - a/(v*v).                                    
*/
float 
vander( float v, float temp )                                                     
{   
    float p;                    /* LOCAL declaration, not same p as above */ 
    const float a = 1.474E+05;              /* constants for CO gas */
    const float b = .0395;         

    p = R * temp / (v - b) - a / (v * v);   /* pressure in Pascals  */ 
    return p / 1000.0;                      /* kPa pressure  */      
}