/* ----------------------------------------------------------------------- 
**  Figure 13.32   Gas pressures.                                             
**  Figure 13.33:  Calculating gas pressure.                                  
** ----------------------------------------------------------------------- 
*/
#include "tools.h"

#define R  8314           /* universal gas constant (N m / K mol) */
#define V  1.00           /* Use volume of 1 cubic meter  */   

void work( void );
        
void main( void )
{
    char do_more;    
    banner();                                  
    puts("\n Gas Pressure Calculation");
    do {
        work();
        printf("\n Do you want more calculations (y/n)? ");
        scanf( " %c", &do_more );
    } while ( tolower( do_more ) != 'n' );

    bye();
}

/* --------------------------------------------------------------------------- */
void 
work( void )
{
    int choice;              /* for the gas menu selection */
    float temp;              /* temperature of gas, Kelvin  */                
    float mass;              /* mass in kilograms       */   
    float moles;             /* number of kmol of gas      */            
    float pres;              /* pressure, in KiloPascals, to be calculated */

   /* ----- Table of gases -------------------------------------------------- */
    #define N    5     /* number of gases in the menu */
    string menu[N]=    { "carbon dioxide", "carbon monoxide",  
                         "nitrogen", "oxygen", "steam"};                                                   
    string formula[N]= { "C_O2", "C_O", "N2", "O2", "H2_O" };
    float mwt[N]=      { 44.011, 28.011, 28.013, 31.999, 18.015 };
    /* ------------------------------------------------------------------------- */

    choice = menu_i( " Please choose a gas:", N, menu );
    
    printf( "\n Enter the temperature in degrees Kelvin: " ); 
    scanf( "%g", &temp );                                         
    printf( " Enter the mass in kilograms: " ); 
    scanf( "%g", &mass );
    if (temp <= 0 || mass <= 0) {
        printf( " Inputs can't be negative, try again.\n" );
        return;
    }                                         

    moles = mass / mwt[choice];                 /* kmol                         */       
    pres = (moles * R * temp) / (V * 1000) ;    /* ideal gas equation, kPascals */       
    
    printf( "\n For %.2f kilograms of %s \n", mass, formula[choice] );           
    printf( "   occupying %.2f cubic meters at %.2f degrees K, \n", V, temp ); 
    printf( "   pressure is %.3g kPa using ideal gas equation.\n", pres );
}