Answer:
 
    
The degrees of freedom are given by:
 
  
And the p value would be:
 
  
Since the p value is lower than the significance level we have enough evidence to reject the null hypothesis and we can conclude that the true mean is significantly higher than 2.3 ppm the safe limit 
Step-by-step explanation:
Information provided
 represent the sample mean in ppm
 represent the sample mean in ppm
 represent the sample standard deviation
 represent the sample standard deviation
 sample size
 sample size  
 represent the value to verify
 represent the value to verify
 represent the significance level
 represent the significance level 
t would represent the statistic  
 represent the p value
 represent the p value 
System of hypothesis
We want to verify if the true mean is higher than 2,3 ppm, the system of hypothesis would be:  
Null hypothesis: 
  
Alternative hypothesis: 
  
The statistic for this case is given by:
 (1)
  (1)  
Replacing the info given we got:
 
    
The degrees of freedom are given by:
 
  
And the p value would be:
 
  
Since the p value is lower than the significance level we have enough evidence to reject the null hypothesis and we can conclude that the true mean is significantly higher than 2.3 ppm the safe limit