During the development of a sampling plan for the determination of Cu in tree bark a standard reference material of homogenized pine needles was used to study the effects of sampling variance. 

a) Using the following data determine the variance due to sampling for this method. 

b) If 0.250 g samples of tree bark were used, what mass of sample should be collected such that the relative standard deviation for sampling is ± 5%? 

 

Cu in Tree Bark Samples (ppm)	Cu in Pine Needle SRM (ppm)
114	178
89.0	174
132	180
105	181

94.0

173