import numpy as np

MFe = 55.845 # g/mol
MCu = 63.546

yld = np.asarray([1.00, 1.50]) # 100% and 150% for Fe2 and Fe3
yld

mFe = 0.78 # g
mCu_obs = 0.78 # g

mCu = mFe / MFe * yld * MCu
mCu

err = np.abs(mCu_obs - mCu) / mCu
err * 100 # now in %