def count_mol_weights(mol_weights):
    """
    Count the number of weights in each bin, and 
    return a list of counts.
    """
    counts = [0 for _ in range(9)]

    for mol_weight in mol_weights:
        if mol_weight < 250:
            counts[0] += 1    
        elif 250 <= mol_weight < 300:
            counts[1] += 1
        elif 300 <= mol_weight < 325:
            counts[2] += 1
        elif 325 <= mol_weight < 350:
            counts[3] += 1
        elif 350 <= mol_weight < 375:
            counts[4] += 1
        elif 375 <= mol_weight < 400:
            counts[5] += 1
        elif 400 <= mol_weight < 425:
            counts[6] += 1
        elif 425 <= mol_weight < 450:
            counts[7] += 1
        else:
            counts[8] += 1

    return counts