The molecular weight and formula given on the product datasheet correspond to the chemical structure displayed there. However, molecular weights (and formulae) can vary slightly from batch to batch due to water composition or a change of salt. These changes will be indicated on the Certificate of Analysis that accompanies your product. The changes should not affect the biological activity of your product, but it is important that you take them into account when making solutions, so you should always use the batch molecular weight as stated on the Certificate of Analysis or the vial label.